Read words with oo, distinguishing the long "oo" sound in moon from the short "oo" sound in book.
The letters oo can make two different sounds. Moon has a long, smooth oo -- like the sound you make when something is amazing. Book has a shorter, quicker oo.
There is no rule that always tells you which oo sound a new word uses. The best strategy is to try both sounds and pick whichever one sounds like a real, familiar word.
Mastery looks like: They read both oo sounds correctly and use the try-both strategy confidently on an unfamiliar oo word.
Common stumbles: Using only one oo sound for every oo word instead of checking which one sounds right.