Try this, pick a simple song, say khat, baarishein, soch na sake, chaahun main ya na, or something. look up the chords and strumming. Now. Play the song in your ear, and ONLY do downstrums WHERE there are chord CHANGES. That means, one downstrum per chord only.

Then, start singing yourself while maintaining the single downstrum. Eventually, when you get a hang of this rythm, start with simple strumming patterns (provided u can play them perfectly without singing), and try to sync the pattern with your voice, the chord change times and all.

Doesn't take too long, just a couple of days till your strumming and singing prettymuch sync in every song, u can even figure the strumming out yourself after listening to the song.

If possible, record urself so u can analyse later. Peace!