Some papers such as gift wrap and magazine cutouts will warp and bubble after applying Mod Podge. To minimize the warping, spray the front and back of these papers with an acrylic sealer such as Mod Podge Acrylic Sealer, and let dry. Be sure to test the sealer first on the paper you want to use before continuing with the project. Apply Mod Podge to the back of the paper or to the project surface and press the paper firmly to the surface, let dry completely.
(Do NOT apply a top coat until the paper has dried completely to the surface.) Let it set for about 5 minutes, then recheck your project to see if there are any more bubbles. If there are, press it down again. If the paper still doesn't lay down, take a straight pin to release air under the paper and press smooth.
If a bubble is found after the project is dried, pop the bubble with a straight pin. Then fill a small bottle with Mod Podge and fill into pin hole with a small amount of Mod Podge. Remove and press area down. Re-check area to see if the bubble has returned. If so, apply a little more Mod Podge and repress.