I would have thought that I would have posted about this long ago as I use the "Blind Stitch" for so many things. Maybe, since I have shown a few of my girls how to do it, I assumed I had posted about it before.
This is probably one of the best things to know for mending and hand sewing. If you look at the top photo, I should have closed the hole a little but you can see that you don't see the seam or the stitch thread on the pillow.
Every pillow and many quilts I have sewn, I do this blind stitch once you stuff the pillow form inside the shell or cover fabric. You could sew in a zipper but on a tight cover like this one, you wouldn't want the bulk or the zipper showing so it really isn't the best option for covering a simple inexpensive pillow.
With the sleeping bag, since the rip was on the inside, I didn't worry about doing a blind stitch and also didn't worry about the color of thread since I wanted you to be able to see what I was doing. Hopefully, in the video you can understand what I was trying to convey but if not, I will try to write it out for those that can't watch videos.
First, pick your thread color and needle. For the thread, you can use embroidery floss which may be cheaper and give you a better match for your project. Just use one or two strands of the six floss strands to sew with.
If the fabric is tight weave you want a smaller needle. If it is looser like the pillow at the top, you can use a little larger needle. You don't want to have to "re-thread" your needle so get enough thread on there to go twice the distance of your hole so you don't run out of thread as making a knot and trying to start again can show through.
When starting with the whip stitch, start on the inside of the fabric where the knot won't show when you are finished. Then, pull the thread through to the outside. Don't pull it too close to the frayed or ripped edge or the thread will pull out and you will just make the rip bigger. Give yourself just about 1/8th inch on the side of the rip.
Put the needle down into the other side of the rip and back up the opposite side. You can pin both sides together making it easier to go through both fabric edges at the same time. This is a good idea if the rip is uneven on the edge as you don't want to get ahead of yourself on one side of the rip and end up with more fabric on one edge than the other kind of like buttoning your shirt in the wrong button hole so you have a bunch hanging down on one side.
Continue in this way until you have finished sewing the hole and you will have thread and mending showing but it if is on the underside of a blanket or inside of a sleeping bag, it really won't matter. Tie a knot at the finished edge and then use the needle to pull the ends of the thread through to the inside of the project as you don't want to cut the thread right at the knot or it can come undone very easily. Never cut the knot at the knot.
For the "Blind Stitch" like at the top on the pillow, start the same way with the same amount of thread and starting with the knot on the inside of the project but on this, you need to fold the rugged edge over into the inside about 1/8th inch and put the needle facing the opposite end of the rip right through where you folded the fabric. Just pick up about five or 8 strands of the fabric and then pull. I show this in one of the pictures. Go to the opposite rip edge, fold it under about the same amount and put the needle in on the fold and pick up some strands and pull. It starts to look like you are lacing your shoe. Don't pick up only a few strands as the thread can rip through them causing a hole.
Don't go down to far before picking up more on the opposite side or you can have a gap. It is harder to pin this one ahead of time as you need to be able to get into the seam and fold to pick up your next stitch. I would say to practice on something you don't care about or some scrap fabric before working on something you really like so you can get the feel of how this works.
I pulled open the seam on the red pillow so you could see how the threads look like shoe laces. Once you have repaired the hole, make a knot as close to the finish project as possible. Don't cut the thread more than 1/2 inch to the knot. Once the knot is made, carefully use the tip of the needle to "push" the knot through the fabric so it is on the inside of the project. You don't want to create a hole but just gently pop it through and no one will know you hand sewed it.