Choose the right needle and thread (match button size and fabric thickness).
Thread a needle and tie a knot at the end.
Position the button where you want it.
Mark the button holes on the fabric (if needed).
Push the needle up through the first hole from the underside.
Push the needle down through the opposite hole (keep stitches tight and even).
Repeat for each hole on the button, making several passes (typically 6–12).
If the button needs a gap (for fabric like coats), place a pin or toothpick between the button and fabric while sewing, then remove it after securing.
Finish by sewing through the fabric a couple of extra times to lock the thread.
Cut the thread close to the fabric, leaving a short tail.
Thread the tail under a few stitches and trim.
If desired, secure with a few final tight knots on the underside.