I have used various left-over yarns. All are at least 70% wool. I had a crochet hook size 3.0 mm. My shoe size is 38 (5).
I'm not writing a pattern as such, only giving some pointers.
Sole:I started the sole in the middle. I used a chainless foundation (you can search that on Youtube if you are not familiar with the technique). But a chain works just as well. I had 32 stitches to start with. You crochet round the middle chain, increasing at both ends.
At the stitch markers I started to widen the front of the sole.Used half trebles (British term) or half double crochet (American term).
One row of doubles (Br.) (singles, Am.) round the sole.
Turned the sole round. A row of slip stitches round the sole. This way the sole and the rest of the slipper will look more separate.
Side of the slipper:
Above the blue and green rows are in V-stitch. If you are not familiar with it, you can see the V-stitch chart here (not my site but found that).
After those I started to decrease stitches in the part that goes over the toes. I didn't count stitches, just checked that the shape seems to be right. The decreases are between the two stitch markers. In the rest of the slipper the number of stitches stays the same.
The finished main part of the slipper:
Flower:Started by chaining 4.
The groups of trebles (British term, the American term is double crochet) were made so that you first crochet 4 trebles, take the hook off the stitch, insert it into the top on the first treble as well as the stitch and pull yarn through both.
The last row is a picot edging. The blue yarn is hidden inside the double crochet stitches (or singles in American terms) from one picot to the next.
Finished! If the slipper seems too loose, you could pass a round elastic inside the top row.