I just got back from my annual 5 day backpacking trip and on the last day, I managed to get part of my sleeping bag stuck in the zipper and it tore! I knew if I left it, all the down stuffing would come out and there would be feathers everywhere! Fortunately, the fix was very simple, all you need is a sleeping pad patch!
Step 1: Locate the Hole
If you have feathers flying out of your sleeping bag, this might be more difficult. Fortunately, I was able to fix the tear right after it happened, so I didn't have to spend hours combing over every inch of my sleeping bag. I've had to do that before and it's a huge pain. Shove as many feathers back inside as possible and then smooth out as many wrinkles as you can.
Step 2: Prepare Patch
If you have an inflatable sleeping pad, you should carry a patch kit with it. At the very least, it should come with some some patches and special adhesive. DO NOT USE THE ADHESIVE ON YOUR SLEEPING BAG! You only need the patch! If you put the adhesive on your sleeping bag, it will make the feathers and layers of fabric stick together and be awful. Don't do it! Only use it if you have to fix your sleeping pad!
Pick a patch that is an appropriate size and shape to cover the hole. If you're feeling fancy, you can even pick a patch in a color that goes well with the color of your sleeping bag. The patches we had were gray and yellow, so I used a gray one to blend in with the rest of my sleeping bag.
The patches should have three layers. One layer is like the back of a sticker sheet, the sticky side of the patch will be stuck to this layer. The second layer is the patch itself, which is like fabric and is very flexible which is why there is a third layer. The third layer is on top of the patch and keeps it from getting folded over and stuck on itself or anything else you don't want it to stick to.
For now, only take off the bottom layer.
Step 3: Stick It On!
Once you have peeled off the bottom later of the patch, carefully stick the patch to the sleeping bag fabric, centered over the hole. Try to make sure there are no wrinkles in the fabric as you stick the patch on. If there is actually fabric missing, instead of just a tear, some wrinkles may be unavoidable. Too many wrinkles will make the patch not stick as well and will actually put more stress on the patch over time and it will need replacing sooner.
Once the patch is in place, apply firm, even pressure on it with your hand to make sure it is really stuck down, especially around the edges. Peel off the top layer of the patch, apply some more pressure, and you're good to go!
I've also used band-aids, moleskin and duct tape to repair sleeping bags in the past, but I've found that these patches work the best and you should carry them with you anyways if you have an inflatable sleeping pad.