What's the best way to hand sew a button back on?

The key is using a thread anchor underneath the button so it doesn't pull straight through the fabric. Start by threading your needle and knotting the end, then bring it up through the fabric from the underside right where the button hole is. Before you attach the button, make a small stitch or two in place to lock that thread down - this keeps everything from unraveling later. Then position your button and start pushing the needle up through one hole and down through the opposite hole, going through the fabric each time.

The real trick is spacing. Leave a tiny gap between the button and fabric when you're stitching - maybe a toothpick's width - so the thread can sit in that space and create a shank. This keeps the button from wearing through too fast and makes it way more durable. Go through each pair of holes about four or five times depending on how much stress that button gets. Once you're done, bring your needle to the underside and wrap the thread around those stitches a few times to reinforce everything, then knot it off.

I've sewn plenty of buttons back onto work clothes over the years, and the difference between a button that lasts three weeks and one that holds for months really comes down to that shank and having a solid anchor. Just take your time with it - there's no rush and it genuinely takes maybe two minutes once you get the hang of it.

Do you have thread that matches the button or are you going with whatever's handy? Thread a needle, make a small knot at the end, and push it up through one of the holes from the underside of the fabric - you want to anchor it in the layers underneath so the knot stays hidden. Then just go back and forth through the holes 4 or 5 times, keeping tension consistent so the button sits flush but not too tight. Finish by wrapping the thread around the shank a couple times (the little space between button and fabric) then knot it off on the underside, and you're done.

I've reattached plenty of buttons on gym bags and workout gear over the years, and the anchor method definitely works well. The main thing I'd add is that you want to create a little spacer between the button and fabric so the thread doesn't snap under tension.

Here's what I do: knot your thread and come up through one hole from underneath, then go down through the opposite hole. Keep doing this back and forth - I usually go through about 6-8 times depending on the button size and how much stress it'll be under. While you're doing this, keep a toothpick or matchstick under the button to create that gap. Once you've got enough passes, bring the needle up between the button and fabric, wrap the thread around the little post you've created a few times, then push back down and knot it off underneath.

The wrapping part is what keeps everything from unraveling. Those wraps basically become a reinforced "neck" for your button, and they distribute the pulling force way better than just having thread pressed flat against fabric. I've had buttons stay put for years this way, even on stuff that gets yanked on constantly.