Ive not done this myself, but if I was going todo so it'd be this way:
Personally, Id go with sticky sided paper you can print on a normal A4 sheet, and with that also put a Clear Matt Transparent Self Adhesive Vinyl Laminate sheet over the top, cut them out an slap em' on the side of your PSU. the plastic laminate makes them look a bit more high quality too.
The only other alternative id think is to order it online, but its a bit risky considering you can see what its going to turn out like until you wait however many days/weeks for it to arrive.
Hope it helps!
CPU: i7-4790K 4.00Ghz/OC 4.44Ghz
Motherboard: Z97S SLI Krait Edition
GPU: x2 NVIDIA GTX 970 TURBO
Memory: 32GB Kingston HyperX (2x White and 2x Black 8GB Sticks)
Storage: 500GB Samsung SSD 850 EVO - 4TB Seagate HDD
Case: Corsair 780t