For those who attended Kathy's two-week class on the fundamentals of chant (and we thank her for giving it), she suggested we brush up on solfege. She gave us a handout showing various intervals. Here also is a tutorial that was suggested in the MusicaSacra forum:
There are many other online resources; just do a search on "solfege" or "solfege Gregorian chant".
(P.S. A little bird tells me today is Kathy's birthday!)