Hi Eli –
On the clip you’re working on (on the time line), click on the pan/crop box (little square box near the end ofthe clip.
Then select the Mask option and draw your mask (you may need to select “negative” to make it work right). If your scene is moving around, you may need to do this every few frames (keyframing).
This whole process, while tedious, is not difficult. It is also known as rotoscoping.
