French seams are amazingly strong, so they're great for purses, totes, clothing and all sorts of home decor items! They're especially nice for clothes - no rough edges against the skin.
This is the easiest and quickest way I've found to do them - sewing 1/8 and then 1/4 inch seam allowances keeps you from having to spend time cutting extra fabric off, and also keeps you from having to account for a really large seam allowance.
Step 1: What you'll need:
Step 2: First line of sewing
Sew an 1/8 inch from the right edge.
Step 3: Finger press it open
Step 4: Fold and sew again
Now sew down this seam again, using a 1/4 seam allowance.
Step 5: Press and you're done!
If you're making something that needs to be nice and flat, you can sew the seam allowance down after pressing, but I normally skip that part.
If you have any questions, feel free to ask in the comments below!