I figured this was needed. I'm not as good as explaining stuff as some other people on here, but hopefully this is pretty clear. Since I use MS Paint and Iaza, those are the programmes that I'll mention in my tutorial. Should be pretty much the same for all paint and graphics programmes, tho.
By the way, when you've transparentised your doll, the doll will save as a .gif file. Also, find Iaza here: Iaza.com