David J provided this tutorial:
Rotoscoping is the technique of painting directly onto video frames.
Rotoscoping is feasible for short sections of video (a few seconds - depends on processor power and memory) using a combination of Premiere and PhotoShop (even the light versions).
1. Generate the original clip in Premiere.
2. Cover it with the work area bar.
3. Export as .FLM filmstrip from Premiere.
4. Import the FLM file into PhotoShop. Close Premiere before opening PhotoShop unless you have lots of memory.
5. Expand the size of the frames using CTRL+.
6. Select the first frame on which you want to draw.
7. Create a new layer. NEVER draw on the original layer unless you are prepared to accept that you won't be able to reverse your action. To be able to re-edit your changes, you need to add layers and draw on those instead. This gives the added bonus of allowing you to copy your painting to successive frames in exactly the same relative position.
8. Draw on the first frame.
9. To copy this to the same position on the next frame, CTRL-SHIFT-ALT down arrow. (To move, not copy, omit ALT. To move up, use UP arrow.).
10. Adjust as necessary for the next frame.
11. Work one frame at a time. Unless you create a fresh layer every frame (duplicate layer before copying = CTRL-J), you will not be able to go back and adjust the preceding drawing after you have copied to the next frame. If you have lots of frames, you can easily end up with lots of layers. Merge down (CTRL-E) the layers for frames you have finished working on to keep the numbers under control.
12. Repeat 9 and 10 until done.
13. Finally, save out to FLM format. DO NOT save as a PhotoShop PSD file - you'll only get one frame!
14. Reimport the FLM into Premiere.
15. Render the segment to get it back into the original CODEC.
The Adobe manuals do not give details of rotoscoping key presses, which are vital for using the technique.
The film strip format can use large amounts of disk space. For those who are short of free disk space, it may be useful to make movie with the rotoscope segment back into the proper CODEC AVI format and then delete the FLM files.
You can paint on the area between the frames, but it won't show in the final result.