I've needed to repair a couple things on my husband's suits since he wears one every day to work. He purchased two brand new suits since he got his job and a button fell off of one of them. Thankfully I was able to find where they hid the spare buttons (inside a small breast pocket) and sewed it on. Please pardon the lint in the photos!
When sewing a suit jacket button (or any other jacket or coat button) there needs to be space between the button and the fabric to which it's attached to leave room for the thickness of the other side of the jacket. The amount of space depends on the thickness of the side of the jacket with the button holes. For my husband's jacket I used two toothpicks taped together to use as spacers.
I also used a good, strong, heavy duty thread when sewing. After threading the needle and knotting the thread I stuck the needle through the fabric where the button would be starting from the front of the jacket (so the knot in the thread will be under the button in the end). I pulled the thread all the way through so the knot catches, then stuck the needle through and pulled it all to the front again. I went through the button (if your button has four holes you'll want to sew diagonally between the holes) and pulled the thread almost all the way through. Then I used the toothpicks on top of the button to get the right amount of extra thread and sewed through the other holes. I sewed through the holes a couple more times in each direction, making sure the amount of excess thread remained the same.
Then I removed the toothpicks and pulled the button up so the excess thread was underneath. I brought the needle thread to the front side, but kept it under the button instead of going through it. I wrapped it around the threads three or four times.
When I was ready to tie off I brought the thread around almost as if I was going to wrap it again, then brought the needle through the loop to make a knot. I knotted a couple times, wrapped once more then knotted one more time, just to be sure.
I cut the thread and was done! Nice new, functioning suit.
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