I actually had that same problem, tried using thick paper and tapeing it on but never could get clean lines.
So the trick to what I did actually is I printed it out onto ADHESIVE paper cut out the lettering and then peeled the paper apart and stuck it onto my car so the paper acted like a sticker and was flush aganist the paint (The paper needs to be flush on the paint or else you will have overspary and uneven lines everywhere). Sprayed it and pealed the paper off and came out great. Hope this helps. Any questions let me know