So say you are on the trail, you get something from your pack and *pop* your nylon zipper trears away from the fabric. it may still zip with a gap but it will not be long before it causes you an issues like derailing, the gap gets bigger makes you lose something or have to replace the whole thing. So you could get a new zipper. Granted good luck finding a new one hiking. This is not just for the trail it is also for that back pack, messenger bag or purse etc you are just not ready to let go yet but can't get a new zipper on.

Either way this is no problem when you carry a sewing kit.

Step 1: What You Will Need.

Needle
like color thread (Nylon thread suggested just be sure to have scissors/knife/nail clippers near as it is not a good idea to try break it with your hands or bite it. I am not kidding it will cut you)
a separated zipper.

Step 2: Line It Up

Line up the zipper against the fabric with the rest of the attached zipper. For this zipper, the teeth as you can see are on the fabric some may be right on the edge. Either way line it up and try to keep it lined up as much as you can while you sew. This part is important to getting it to zip up properly.

Step 3: Start the Thread

Pull the thread to an even length on both sides through the needle, also it does not need to be really heavy thread. Put knot in your thread (or if you want use other methods to secure the thread from slipping out) and start from inside. You will want to get your first thread placement right next to or a little under were the zipper should be. That is why lining it up is vital.

Step 4: Sew Between Each Tooth

Now here is the tricky(or tedious) part; guiding the thread through each tooth as you sew. Do not worry to much about missing one as you should go back over it again to get teeth you missed and reinforce the stitches. Use your free hand to keep the alignment and steady the item while working. Keep the stitch tight and the thread from laying on the teeth.It just needs to be between.

Low on thread? Start the first stitch a bit before the separation and loop it through to the end,as you go along making every stitch count you should be fine. Tie off at the end.

Step 5: Almost There...

Keep going between each tooth till you reach the end and a little beyond. Then if you wish go back over it again in reverse (sewing from outside down through the teeth to the inside) stitching past where it had separated/you began. This will make it a lot stronger but if you can not expend the thread do not worry to much about it.

Step 6: Secure It.

I personally sew through a close to the last inner stitch/not visible area on the inside and tie it or use the rest of the thread to sew in one spot repeatedly till it would take cutting i to remove it.