Find the spare, then compare it
Check the garment's inside seams and any small packet saved from purchase. A spare button may be attached where it is easy to overlook. Compare its size, thickness and fastening style with the remaining buttons. A button that is nearly the same colour but too thick can be difficult to pass through the buttonhole.
If there is no spare, take the garment or a matching button to a haberdashery. Decide whether an exact match matters in its position. A visible front fastening may need a close match or a complete set changed together. An internal button can often be chosen for function, provided it works with the fabric and hole.
Inspect where it came away
Look at the cloth around the original attachment. If the fabric is torn or very thin, sewing a new button onto the same weakened spot may not hold. A repair professional can assess whether reinforcement or another repair is needed. Do not pull at loose threads to see how far they go; you may undo stitches that are still doing their job.
Notice how the neighbouring buttons are attached. Some sit on a small thread shank to leave room for the overlapping fabric. Others have a built-in shank. Thick coat fabric needs a different arrangement from a fine shirt. If you have not sewn this type before, practise on scrap fabric or ask someone who repairs garments to demonstrate.
Keep the task small
Gather the matching thread, an appropriate needle, scissors and somewhere to hold the small pieces. Work in good light with the garment laid securely rather than balanced on your lap while moving around. Keep needles and loose buttons away from children and pets, and account for the needle when you finish.
After the repair, fasten and unfasten the button gently. Check that the overlapping fabric lies as intended and that the button is secure without being pulled too tightly against the cloth. If it is difficult to use, the attachment may need adjusting rather than more force.
Save any remaining spare with a note identifying the garment. A tiny labelled envelope takes less time than searching through a jar of unrelated buttons next year. The repair is complete when the fastening works normally again, not merely when the missing space has been filled.
