I knew it was possible to make slippers from garter stitch squares and my child had stated 2 things - that he needed new slippers and that he should learn to knit. It seemed like a great idea to teach him to knit garter stitch squares and make his own slippers.

After studying many 8 square garter stitch knitted slipper instructions, all you really need is to know the length of the foot you’re knitting for.

Half foot length = diagonal of squares.

Use this to calculate your side length from your diagonal length https://www.omnicalculator.com/math/square.

Knit 8 squares (you can totally knit 6 in a long strip)
The two squares hanging off the sides need knitting (or sewing on) with their rows perpendicular (at 90 degrees) to the line of 6.

Use one of the assembly guides in the patterns I’ve linked this project to.