I should probably start by specifying this is a tutorial for how I do a hand stitched blind hem. I've come across a few other methods in which the some of the details are tweaked just slightly but this is what has been working of me for years. This can also be done on a sewing machine (if it has the proper stitch option) but I enjoy slowing down and doing these finishing touches by hand. It isn't only limited to the hems of garments but can be used to finish arm holes or collars. As long as the hem allowance isn't too big, it works on a curve as well. (For example, I have about a half inch allowance for the arm hole project below.)
For things that endure a lot of wash and wear, like pajama bottoms or kids clothes, it's best to stick with a simple, sturdy machine top stitch. But for things like dresses or skirts or items made with finer fabric, I like the look this blind hem stitch offers. Sometimes I come across a lovely handmade dress but the puckered, machine top-stitched hem is an eye sore and can ruin the whole look. Another thing this hand stitched method offers is that you can pull it out and drop your hem later without a lot of hassle or mess.
If done right, the stitches from the outside should be barely visible.
Start by gathering up some pins, a needle, scissors and thread that matches your fabric.Fold back and pin half your allowance. (In my case that would be 1/4". I just eyeballed it but you can measure and mark on the wrong side of the fabric first.) Fold back again, the other half of the hem allowance, and pin. All your raw edges should be tucked inside and no longer visible. From the wrong side of the fabric, pinch the folds and pin. This photo shows what it looks like from the outside/right side of fabric (above) and what it looks like from the inside/wrong outside of fabric (below.) With your needle, threaded single, knot at the end and the wrong side of the fabric facing you, stitch through the bottom fold and the top fold as seen, at perpendicular angle. Be sure that when you go through the bottom fold, you only go under/catch a few threads of the fabric. That stitch is the one that can be seen from the outside. You want to hit that sweet spot of just enough to be sturdy but not to much that it's super visible from the outside. When going through the top fold, you can catch more. Keep doing this every half and inch or inch. Here is what it will start to look like when you're holding it. Here is what it looks like from the inside/wrong side of the fabric, when you've removed the pins and flattened it out. And here is a shot of how it looks when done: the top shows shows the inside view and the bottom shows the outside view. You can see, or rather can hardly see, the stitches. A little press with the iron will smooth out any indents.