Avoid air bubbles by applying the wallpaper properly. The advantage of non-woven wallpaper is that if it does not sit well, you can easily pull it back (it does not tear easily) and you can re-apply it. Rub the air bubbles towards the edges with a wallpapering spatula. If there are small air bubbles because there is no glue behind, use the glue syringe. You can remove an air bubble by pricking or stabbing a small hole in the wallpaper with a needle or the tip of a Stanley knife. Then press the air out of the bubble. With a glue syringe you can, if necessary, squirt a little glue into the bubble. After gluing, check that there is no air bubble.