Genius slipper design where you knit a short "scarf", fold and sew onto your bottom. Garter stitch extra chunky yarn about wide, and however many rows you need, then sew into the 2 layers of the felt bottom.
Beautiful Puff Stitch Patterns I Can't Wait to Try
If you're curious how to crochet on flip flops, this post will answer all your questions including if they fall apart over time. Great free tutorial on how to get started crocheting shoes and boots for spring, summer, fall, and winter.
Sapatos de Crochê, como fazer - Inspiração, tutoriais e referências em Crochet Shoes
This pattern is for knit slippers, but I can totally see using a wool sweater and cutting and serging slippers from it! Home Slippers - DIY. this is knitting, but I bet I could use this to crochet some