Make sure to get rid of all bubbles under the wallpaper using a wallpapering tool. A wallpaper brush is often too soft for this; use the recommended wallpapering tool instead.
Stubborn bubbles that cannot be removed may be due to there being insufficient or no paste. There may also be old bubbles in previous layers of old, paper-based wallpaper that are swelling. Often these do not expand once the paste has dried, but if they are bigger than the size of a palm they should be sanded and filled before continuing.
If you have miscalculated and find the odd air bubble after the wallpaper has dried, these can be dealt with as follows:
Make an incision through the entire bubble using a sharp razor blade. Try to lift up the paper and get paste inside the bubble. Press it down and allow the paste to dry.