A clean finish is a great way to nicely finish seams without a zigzag stitch or a serger. It looks nice and professional also. It can only be done with medium to light weight fabrics or else it gets too bulky. It does take longer so I only do it on nice skirts or jackets that I don't want to line.
First, sew a 5/8" seam.
Iron it open.
Trim each side to 1/2".
Fold ends under towards the seam and iron.
It should look like this. It will have creases at 1/4". Don't worry if it doesn't stay folded.
Making sure you're only sewing on the seam allowance, sew 1/8" from the fold. You're sewing the fold down.
One side done
The is what the underside of the fold looks like.
Iron open, and there you have it.
Now what happens when you sew it to another piece of fabric?
Pin to the corresponding piece. Pin the clean finished seams open.
Sew the seam.
The sewn seam.
The clean finish looks nice when kept open and keeps the bulk from building up.