Short and simple answer: Don't use Rotobrush and do it with conventional masks.
Is there a long and difficult answer?
Is there a tutorial on creating a conventional mask for what I'm trying? I'm a noob.
Scott Squires put together the best rotoscoping demos I've ever seen. He has done a bunch of this and his basic techniques are just what you're looking for.
There is one caveat, however. The demos are done with Commotion (no longer available but the best roto app ever. I keep an old Windows machine with Commotion installed in the garage just in case I ever have a big and involved roto project that isn't easily handled by AE or Mocha) There are not specific instructions, but rather technique and it's exactly the same in AE. You use the pen tool (keyboard shortcut g) to set mask points, enable mask keyframing by selecting your layer pressing Alt/Option + m or by simply pressing m to reveal the mask property and click the stopwatch. Then you start adjusting your mask controls.
You'll find links to Scott's articles here.