what is the "recipe" for glue and water to decoupage some art pictures to some wood?

Answer 1

If you go to a craft store you can buy Mod Podge and that works really well. You can get matte or glossy, personally I prefer glossy and I use a foam brush to apply it. The mod podge is about $6-8 for a decent size bottle and the foam brush is about 25 cents, I usually get two or three.
I also found this recipe on http://www.theholidayzone.com/recipes/glue.html
Decoupage Glue
white glue
warm water

Combine 3 parts glue with 1 part water. Mix thoroughly. Store in air-tight container.

Answer 2

i think its a 1-1 ratio. they make a product just for decopauge, i think its called modge-podge. You can get it at craft/fabric stores