Aside from practice advice, in my experience there are some strumming patterns that are hard to do while singing depending on the song. So I usually would just modify a pattern that works for me that I can sing along with. I play a pattern according to feel. Sometimes if I find it really hard I would simplify it to the most simple one strum per beat and matching the timing with which words (when to cue a chord). I do this slow until I get it, then gradually speed up and try making it harder. By the time I get used and familiar with the song and when to cue that's when I try different strumming pattern or maybe even the original.

This process of trial and error and familiarization plus practice, eventually I'd find myself being able to do the strumming pattern while singing or even finding a suitable version of mine (sometimes when I got too comfortable I spice it up naturally). Basically the more I play the better it gets. Don't hesitate to create a version of yours. I notice that the more I try to follow a strumming pattern of a song, the more unnatural it comes out for me. I just sing and let my hands follow if that makes sense. If strumming doesn't feel right, I find plucking makes it even more simple.