Of course there is a ton of advice waiting for you in the archives with a search for ‘roto’.
But basically, Start at the first frame of your clip and use the pen tool to draw a shape around your subject. Turn on your keyframe burron on the ‘mask shape’ parameter so it will start keyframing your changes. Now go to the last frame of your clip. Move and readjust the points on the mask to reconform it to where your subject is now. Now go the the very middle of the clip and correct it there. Then go to the 2 points in between the 3 keys you now have and correct the mask some more. Keep halving the time between keys and readjust. At some point the mask will be close enough.
Additional tips:
-Don’t try and make 1 mask work for the whole person. Use one mask for the head, another for the torso, etc.
-If you are rotoing only to get a general shape for a cutout effect, don’t go crazy trying to get a flawless roto. You only need a flawless roto when you are pulling an object out of one environment and trying to seamlessly put it in another.
-use the ‘rotobezier’ option when after clicking on the pen tool. It uses B-Splines instead of bezier which in my opinion is far easier to deal with than bezier when animating masks.
Chris Smith
https://www.sugarfilmproduction.com