Open After Effects and create a new Composition (Composition > New Composition...) set to the same pixel size, time length and pixel aspect ratio as your footage you want to rotoscope.

Import your footage into the Composition by going File > Import > File...

If necessary, in the switches panel, set the footage's display quality to best.

In the timeline, double click the footage's name so it will present itself in it's own footage window.

In the toolbox, click the Pen tool. (If the toolbox is not visible, go Window > Tools)

In the new window for the footage, draw a mask to define the area of your lightsaber by clicking each corner of your lightsaber refrence prop and then clicking the point you first created to close the mask. (If your lightsaber prop is not visible in the first frame of your footage, draw the mask off of the footage's canvas)

Click the triangle next to your footage's name in the timeline window, and then click the triangles next to Masks and then Mask 1.

Click the stopwatch next to Mask Shape. (This allows us to set keyframes for the mask we just created so it can change shape over time)

Using the Time Controls palette (Windows > Time Controls) move forward through time to a frame where you want to add a new keyframe.

In the toolbox, click the Selection tool (represented by a standard cursor arrow) and then click anywhere off of the canvas in the footage window to deselect all the currently selected points on the mask.

Click (hold) on the corner of the mask you want to move first and drag it to to the approproate corner of the lightsaber refrence prop in this new frame in time and release. Do the same to the other corners of the mask so that the mask once again defines the area of the lightsaber refrence prop. (If your saber was originally out of view, make sure to go back one frame before your blade is visible and add another keyframe of the points off of the visible canvas. Otherwise the saber will gradually move to the new kayframe over time)

Tip: At this time you should use the Time Controls palette to move back through time over the course of your animation to ensure that the mask fully surrounds the lightsaber refrence prop on every frame between the keyframes you just set. If the mask does not move how you want it, you can set more keyframes by moving to a frame in time where your mask needs the most correction and simply update the mask by moving the corners to where they should be.

Again using the Time Controls palette, continue moving forward through time, setting keyframes to update your mask as you go until you have keyframed your lightsaber over the entire duration of your clip. If you come to a time where your lightsaber is obscured or blocked by something in a way that "breaks it in two" requiring two different parts of the lightsaber to be seen (for instance, swings behind your actor's arm) for now just draw the saber as if you could see right through the thing/person to the whole lightsaber. We'll handle this problem once the rotoscoping is finished.

Note: For frames where your lightsaber is completely obscured or out of frame so that none of it is visible, position the mask completely off of the footage's canvas. If the lightsaber is out of view for more than one frame, you can move the mask off the canvas, skip ahead to the next frame the lightsaber is visible, update the mask, then move back in time one frame and reposition the mask to be off of the footage's canvas again in the same area it was before.

Go back to the beginning of your footage and move forward until you come the first frame that something blocks or obscures your lightsaber that causes it to be "broken" and then move back to the frame right before it.

In the toolbox, click the pen tool.

Draw a new mask off of the footage's canvas with 5 or 6 points. (If you know what you're doing, you will know how many or how few points you will need here, 5 or 6 is just a generic number of points to use for this example)

In the switches/modes panel of the timeline window, set the new mask's mode to Subtract.

Click the triangle next to the new mask's name in the timeline window, and then click stopwatch for Mask Shape.

Using the Time Controls palette (Windows > Time Controls) move forward to the next frame. (This should be the frame where the lightsaber is first "split in two")

In the toolbox, click the Selection tool and then click anywhere off of the canvas in the footage window to deselect all the currently selected points on the mask.

Move (click and drag) the points on the new mask to define the area of the lightsaber mask that should not be seen.

Using the Time Controls palette, continue moving through time and keyframe the new mask to hide the part of the lightsaber blade you do not want seen.

When you come to the frame where your lightsaber should no longer be obscured, move all the points of the subtration mask off of the canvas. (Make sure that the frame before this one has a kayframe set for it)

Close the footage window and create a new Composition (Composition > New Composition...) set to the same pixel size, time length and pixel aspect ratio as your first composition.

Add a new solid to the new composition by going Layer > New > Solid...

In the Solid Settings click the "Make Comp Size" button to ensure that the sollid will fill the canvas space.

Click the color swatch, set the color to pure black and click "OK".

In the Solid Settings click "OK". After Effects now inserts the solid into your new composition.

Add a another solid to the new composition by once again going Layer > New > Solid...

Click the color swatch, set the color to pure white this time and click "OK".

In the Solid Settings click "OK".

If necessary, in the switches panel, set the display quality of the solids to best.

In the timeline, click the Comp 1 tab to display it's content.

Click the name "Mask Shape" under your footage's Mask 1. This should highlight all the Mask Shape keyframes for Mask 1.

Copy the mask keyframe data by going Edit > Copy

In the timeline, click the Comp 2 tab to display it's content.

If it is not, position the time marker on the very first frame of the composition. (By default it should be there already, but in case it isn't this is very important)

Click the name of Solid 1 (Your white solid)

Paste the mask keyframe data into Solid 1 by going Edit > Paste

Apply a blur effect to the saber core layer (Solid 1 layer) by going Effect > Blur & Sharpen > Fast Blur or Effect > Blur & Sharpen > Gaussian Blur.

In the Effect Controls window, give the layer a blur of 1 or 2 or maybe even 3 depending on how close or far the lightsaber is from the camera and then close the Effect Controls window. Remember, this is how blurry the core of your lightsaber will appear, so don't over or under-do it.

Click the triangle next to Solid 1's name in the timeline window, and then click the triangles next to Effects and then Fast Blur or Gaussian Blur.

Duplicate the lightsaber layer by going Edit > Duplicate

Click the name of the #2 Solid 1 layer you just made and then set it's blurriness to 10.

Note: In the end we will have 3 lightsaber aura layers with varying blur values in a 1:2:4 blur ratio, as I call it. For the example of this tutorial I am using 10, 20, 40. This set of blur values will not look ideal at all times. You will have to playa round with these numbers to find what glow works for your shot. For closer shots you might use 20, 40, 80 or 30, 60, 120 and for far away shots you might use 5, 10, 20. The point is as long as the blurs are in proportion to each other, you have to tweak them to make sure they look as good as they can for your shot.

Duplicate the layer again by going Edit > Duplicate

Click the name of the #3 Solid 1 layer you just made and then set it's blurriness to 20.

Duplicate the layer once again by going Edit > Duplicate

Click the name of the #4 Solid 1 layer you just made and then set it's blurriness to 40.

In the timeline, click the Comp 1 tab to display it's content.

Click anywhere in the timeline to deselect anything currently slected, click the name of your footage's Mask 1, and delete the mask by hitting the delete key.

Position the time marker on the very first frame of the composition.

"Nest" your lightsaber composition (Comp 2) into Comp 1 by clicking the Comp 2 name in the Project window and dragging it to the center of the Composition window. (The composition should snap to the center of the composition window, ensuring that it is lined up with your footage)

If necessary, in the switches panel, set the display quality of the nested Comp 2 to best.

Set the transfer mode of the lightsaber layer (the nested Comp 2) to screen mode by going Layer > Transfer Mode > Screen

Tip: If you want to add a nice roundness to your lightsaber corners/tips, now is the time to apply a median filter. Make sure your nested lightsaber composition (comp 2) is still selected and go Effect > Image Control > Median and apply a median radius of 1 or 2 or maybe 3 depending how large your lightsaber is in frame.

Go Effect > Adjust > Color Balance

In the Effect Controls window, first click the Preserve Luminosity checkbox and then give the lightsaber color by punching up the Midtone and Highlight Balance for the color you want.

Note: Colors other than red green and blue can be created by punching up the midtones and highlights of more than one color. Purple = Red + Blue, Yellow = Green + Red and Cyan = Green + Blue.