First make a Gluing Screen.
To make a gluing screen, you can use either a square of soft plastic window screen cut large enough to cover your entire project surface. You could also use nylon net, but with nylon net you may need to make it about 4 layers thick, and stitch around the edges to hold all the layers together.

Sticking the prints down.

Using Decoupage Paste made according to the recipe below, Apply a generous amount to the back of your print. Take your time and allow the paper to soak up the glue. After the paper has soaked up enough glue to satisfy it's hunger, you must have enough excess to adhere to the item you are working on.

Place the glued print, glue side down on your project, and gently position it where you want it to be. Place the gluing screen over it and rub the print from center to the edges (through the screen). When you are happy that there are no very large air bubbles remaining between the object and the print remove the gluing screen, take a damp sponge and gently remove any excess glue that has seeped out from under the print. Replace the screen and use a rubber roller and start rolling the print back and forth until it is absolutely and completely flat. Keep checking and rolling, check and roll. Do not rush this process as some paper tends to "pop up" if the glue has not taken hold properly. I usually roll my prints down for about a minute or more.

Prints may be layered one on top of the other to form your design. You can keep adding prints until you are happy with the results. This is where having lots of cut designs on hand is really good. That way you do not have to go back to cutting before completing you gluing session.

HANDY HINT FOR GLUING

When you have made a temporary layout of your design on the object, draw a dotted outline around the prints with chalk to use as a guide for positioning the prints when you finally get down to gluing them on permanently.