I've made P a few pairs of underwear now, and I thought I would share my methods with you, since they differ slightly from most of the tutorials I looked at.
Cotton knit fabric - I used some old t shirts to cut up, bonus points if you can include some kind of cute logo for the undies
Note: For hygiene's sake, the crotch piece of the underwear should be cut from 100% cotton, even if the rest of the undies aren't.
Elastic - I used regular old 5mm braided elastic, but this is a little bit hard on the skin, so next time I'll try and find some softer underwear elastic or FOE.
Most of the tutorials recommend that you cut up an existing pair of underwear to create the pattern pieces to work from. This is a really good idea for a much loved and worn out pair of undies. But the frugality in me balks at cutting up one perfectly serviceable pair of P's undies in order to create another perfectly serviceable pair of undies. So instead I drew around P's undies as best I could without cutting them up.
You will end up with 3 pattern pieces - a front piece that looks like a curved T shape, a back piece and a kind of triangley squary shape for the crotch piece.
Cut out your fabric, making sure you lay the pieces so that the stretch of the fabric goes across the pattern
I didn't take a photo of this stage, but you all know how to lay out a pattern right?
Note: I use my overlocker for this entire project, but if you don't have one, a zigzag stitch on your sewing machine should do the job equally well
Note: It pays to leave a bit of a gap between the top of the fabric and the elastic - this helps to prevent the elastic touching skin. If you are overlocking, DON'T cut the elastic as you go, the gap at the top of the fabric helps with this as well. Just go carefully.
Once you have sewn up the sides, the crotch piece should also be attached, with just the finished top edge not attached to the undies - it should remain pretty secure.
Once all the elastic is attached it should look like this.