Perhaps the most common of customs, altering action figures are what most people think of when you mention "customs". The availability of cheap fodder, or parts is probably the major reason for this. As well, completion of these projects can be as quick as 20 minutes giving the creator a swift sense of satisfaction (as with this simple repaint of a vintage Leia Hoth).
Parts - you don't have to stick to Star Wars action figures for your fodder. Keep an eye open for other figures in the same scale when they go on sale. Especially look for female parts ... the variety of female heads is limited in the Star Wars line. Also, look for figures with lots of articulation (movable joints) so you can pose your figures! After a while, you'll be a master at determining what parts will be just right for your next project. Disassembly and re-assembly - The main technique to master is this one. Unless you are doing simple repaints, you will want to experiment with switching parts on your figures to get the look you really want. You have reference pictures of the character you want to make - great. Looking over your stock of figures, you may wish to use the head and torso of one, and the arms, legs and pelvis of another. The first thing to do is remove the head of both figures. This is done by the "boil and pop" method. Boil a small amount of water in the microwave. CAREFULLY dip in your figure, head first and keep it there for a few minutes to soften. With a towel wrapped around your hand, gently but with continuous force, pull on the head. If it's soft enough, it should "pop!" out. If not, return it to the water to soften some more. Most heads have a long neck stem, so it may look like you're stretching the heck out of it, but it will return to normal shape when cooled. Try not to force it too much or it will tear, and you won't be able to use it again without some reconstructive surgery.
Once the head is out of its neck socket, use pliers to pry apart the torso. You may want to soften the glue by dunking the figure back in boiling water, but be careful! Not only will hot water come gushing out, the torso may suddenly snap and parts will fly! Wear safety goggles to protect your eyes.
If you are successful in getting the torso apart, you can disassemble, then re-assemble the parts. Dry fit them first, to make sure everything fits before glueing back together. You may need to do some alterations like carving down posts so that you still have movement. Now you can repaint, or add new features using sculpey!