Masking in After Effects

o.k let's talk about workflow.

1. how long do I have to Roto?

first thing you should do is determine how much of this footage you need to be rotoscoped. set the in and out point of the frames you want to roto. rotoscoping is a laborious process and you want to work as efficiently and as little as possible. this is a preparation step you can take in your editing software or in Ae. trim your footage to only the part you need to handle.

for example here I have trimmed the footage and the duration to only 4 seconds I want to work on

2. how many pixels do I have to Roto?

next step is determining how much pixels you actually have to roto. do I really need the whole person including the desk to appear on a different background? or do I just want to place another image on the background which means I only need to cut a smaller part of the person. here for example I set guides to show me what I need to roto. I have scribbled here in green - the part I want to roto, in red - the new background

3. Choose a Technique:

to handle a cut-out of an image from a noisy background you can:

1. use masks with keyframes: https://helpx.adobe.com/after-effects/using/animating-shape-paths-masks.html

read carefully the rotoscoping tips in the help files. there is more to it

2. use Mocha that allows you to track and roto the person in one step: mocha AE Creative Cloud Rotoscoping Demo with Mary Poplin at SIGGRAPH 2013 - YouTube

3. use Rotobrush: the workflow is described in the links I sent earlier

here's a interesting article: https://www.fxguide.com/featured/the-art-of-roto-2011/

as you can see - this takes a lot of work.

Good luck