I agree with Andrew and Sam. The water/soapy water way with squeegee thing is the best method. I have done this several times to my car personally, banner stickers and smaller stickers and it is the best method I've known. First because it's easier to put on but also, since you have a little bit of soapy water between the glass and the decal, you can adjust the position slightly if needs be without worrying. Then just squeegee the moisture away and with a microfibre towel or something, take care of the excess liquid and stuff.