Hi and welcome to CC. I'm going to move your topic to the customizing forum, because it is primarily where questions like this are answered.
I can answer a few questions.
www.bricklink.com is a place to get blank torsos most of the time. Some people remove their decals from LEGO piece using a substance called Brasso.
As far as the material different people use to print their stickers, well this varies. Myself, I am not a customizer at all. But when I'm in need of a sticker, well I use window cling material and run it through my printer. Why window cling material? Because it sticks by static only. I can remove it without destroying my pieces (I am NOT a fan of the destruction of pieces). And I can put it back on if I so choose. As far as I'm aware, I might be the only person using this method.
There are some using sticker sheets that run their their printers and still other use something called waterslides. But I'll let the experts help you with those.