Ok so I need to sew up the seam of the outer sole. Then it says to "short backstitch" the two soles together, but I'm not sure what that means. Can someone describe it or tell me how you did yours? Thanks!

Just sew them without a lumpy seam. The felting will hide whatever you do.

I used an edge-to-edge seam to avoid a lump.

http://www.knitting.co.nz/site/page_affix/toaffix_learntoknit13/

I agree with Ingrid. Just do it.