Good, sharp, lightweight scissors
Here's a basic "strand" of hair. Basically, cut the pieces as you go along, and layer them on the head
This shows what you should and should not do. When you cut the pieces too symmetrical, it looks off, and unnatural. Try to cut them in a more organic fashion.
When glueing it to the head, always start from the bottom, and then work your way to the top. Your plushies will resemble old men with horn-shoe hair...or horrendous mullet-action will also be going on...but bear with it!!
This hair was entirely created with these simple strands. It always depends on the reference; in this case, his hair didn't require much volume.
(this was the reference picture I used)
When you cut all of your pieces according to Case 1, you'll notice that there's a great deal of scraps/wasted pieces. You can either reuse it, or if you're short on felt, you can use the long method...
This "method" consists of cutting the pieces individually so no pieces are gone to waste. Honestly, it's LONG!
You have to glue every single strand... it's tedious, but it works!
Here's an example where I glued every single piece. In this case, the strands were short triangles, but it's the same concept. It took at least 2 hours to construct, so be prepared to invest your time!
To get volume, a variety of "folds" are required. When you fold a piece of felt, and glue it into shape, the piece becomes stiff/more solid. It will therefore stay in place. Here's a very simple case: just cut a regular piece that has a pointed tip on both sides.
Now, twist it once, like shown in the picture. Glue the spot where my fingers are pinching it into place (i.e. in the center).
Here's an example of how you would apply it to a head. Renji's ponytail was created with this technique.
Here's another way to create volume by using a "stiff" strand. Cut a piece that is blunt on one edge, and pointed on the other.
Fold both corners; glue into place.
Here's an example in which I used this technique.
To create what I call "chunks" (hair that's in large strands, like tentacles!), cut a piece like so.
Fold the flaps together and glue into place.
Here's an example of what it can create.
You basically have to play it by eye, and use whatever techniques suit the situation. In this example, I used case 1 and 5. I can't really explain how I know which techniques to use; I just stick to my reference picture, and try to make it look similar.
A good reference works a long way. It helps to know how the hair is in fact parted/styled. Only one picture of the hair is often not enough; you'll generally need a 3/4 view and a front view at the least.
Finally, craft felt is sort-of fragile. If the piece is too small, it will break off. Also, if the plush is going to be hoarsely handled (e.g. by children), it's not a good idea to use felt. In any case, you can get really nice results, so it's worth a try if you're making a display plushie. Good luck! ^_^