The words are chosen from text she wants to or needs to read: i.e. they are most of the new words in a particular piece and include words I know she has struggled with before.
What I find very satisfying is that Tammy is now better able to split up a word in syllables and read it syllable by syllable instead of just guessing the word. When we come to a difficult word, I write down the "bits"and we practice saying it, bit by bit. Not only does it help much with pronunciation, but I find that in the end she can actually spell words like inevitably, mischievous, initial, relationship. (Bearing in mind that she is supposed to be mentally handicapped, this is quite an achievement.)