Double click on the layer so that you have the video you want to rotoscope active and showing in the panel
With the roto brush, select the inside of the object (not the edges) you want to select. See the green line.
If it selected too much, option-drag an area you want to subtract. See the red line.
If you want to refine the edges, use the refine edge tool and drag on the edge
Go forward few frames and incrementally refine the mask as you go
Once you are done, (or when you are ready to save to a file), make sure you hit the
freeze button. Then the state will be saved. (If you do not hit the freeze button, your effort will be lost)