Awesome, I get it now! I just knitted the whole welted band and all looks pretty good so far, so I think I was able to purl into the back purl bump correctly.
The link to my pattern is here:
It's all just decreasing until the end from now on so should be smooth sailing from here. I'll post a picture of the finished product if I can get it to felt properly. I've never felted anything before so we'll see.
Thanks again, you've been a big help!