Looking for free knitting patterns for slippers? The first five slipper patterns use garter stitch because it produces a thick squishy texture.

Three of the patterns are Youtube tutorials without any written instructions. I think they are quite easy to follow so I decided to include them. I may transcribe them later when I have time. 🙂
Simple garter stitch slippers

Designed by Hanna Levaniemi.
I tried this pattern and really enjoyed it. The slipper starts out like a garter stitch square with a little shaping for the toe. The heel is formed by seaming up the cast-on edge. I think I can substitute this with a Turkish cast on and continue with knitting back and forth.
The instructions could include a few more pictures on how the slippers are seamed together.

Seam up the toe and crochet the 2 sides of the squares together to form the instep. Quick and fun.
This pattern is available at https://www.ravelry.com/patterns/library/simple-garter-stitch-slippers
Slippers with 5 leaves

Designed by Sercan Senay.
This pair of slippers is quite similar to the simple garter stitch slippers. The difference lays in the shaping of the toes. This one is done using short rows.
This pattern is available at http://stitchland.blogspot.com.tr/2014/11/yaprakl-patik-orelim-mi.html
South Marysburgh slippers

Designed by Sam Lamb.
Unlike the previous slipper patterns which are in garter stitch, this one is in stockinette. I like the shape a lot. It requires the knitter to know how to pick up stitches and knit short rows.
The instructions were surprisingly easy and it is a quick knit. Gauge is important. Do take time to check gauge.

This pattern is available at http://samlamb.blogspot.com/2014/11/south-marysburgh-slippers.html
Seamless Salomas slippers

Designed by Susan Busbee and Megan Williams.
This pair of slippers is made up of a combination of ribbing and garter stitch. Easy to knit and really stretchy.
This free downloadable pdf is available at https://www.ravelry.com/patterns/library/seamless-salomas-slippers
Free Knitting Patterns for Slippers Video Tutorials
Cotton garter stitch slippers
Designed by the Pattern Family.
Many knitters learnt how to knit using Youtube. I found this video tutorial for a garter stitch slippers. The difference between this one and the first two is the way the garter stitch is used.
Unfortunately, the designer did not write down the instructions. I may transcribe it if I can find some time. 😛
Garter stitch slippers with a crochet edging
Transforming garter stitch rectangles into sweet slippers.
Again, I did not find any written instructions for this video tutorial but there is an English transcription for the video.