Marc Spess has created a comprehnsive tutorial on this subject which can be bought at the store..
http://www.stopmotionstore.com/index.ph … ducts_id=5
You can also look on the net for examples of phoneme mouth shapes to match to your dialogue. When i need to make quick and easy mouths, i create the seperate mouth shapes in super sculpey of similar clay and then simply stick them to the character. You could also use simple print outs of mouth shapes used for example in the kids show "rotten ralph" in the UK. Theres a variety of different ways to go about it , but for a start try typing in "phonemes" into google and have a look at the mouth shapes.
Hope this helps