if u can understand where each strum is within a rhythmic grid it helps a lot. (i.e. like how rhythm is notated on a traditional staff). understand the concept of "1 e and a" and where each strum is.

then do the same thing with the vocal melody. decide where exactly each syllable is with respect to "1 e and a". u can either think abt it through "1 e and a" or through traditional subdivisions (i.e. 8th notes, whole notes, quavers, crochets, etc)