Usually a spray bottle filled with water and add 2 to 3 drops of liquid soap is all that is required.

From a decal site:
Wet Method
Be sure that the surface is clean and free from residue. Find center of decal and the surface on which you want to apply it. Hold the decal in place desired and mark two top corners with pencil.

1. Put 2 to 3 drops of Ivory Liquid per gallon of water and sponge onto adhesive surface only. DO NOT wet backing sheet. (Glass Plus or other non-ammonia window cleaner may also be used).
2. Start separating the decal at one corner and pull it back slowly, keeping the adhesive wet as you go.
3. Wet the surface with the same solution mixed previously and position the decal to the marks established in No. 1. Wet hands with solution and rub over decal lightly, smoothing out any wrinkles.
4. Work out water with plastic squeegee and dry excess liquid with a soft cloth or paper towel. Squeegee the decal from top to bottom beginning at the center. Proceed right from center, and left from center overlapping strokes 50%. ***** any air bubbles with a pin and rub down. Be sure that outer edges of decal are rubbed down smoothly and all liquid is forced out. This is essential for proper adhesion.
5. Decal must dry out for 24 hours before adhesion is complete.