The pick tapping must be done close to the fret but not on it. Just like when you fret a note with your fingers. You also have to control your attack. Since the pick is narrower than your finger, less pressure needs to be applied. Tapping too hard might make the note go out of tune.

You need to stay in the same key when you tap. The note you tap must be in the scale you are soloing with. Most of the mistake I see from guitar player starting to incorporate tapping is that they'll tap random notes. You must stay in the scale you are using :)

Hope that helps you out.