Getting the strumming right is the most important thing in the first place. Most students I encounter are a bit over-confident about the quality of their strumming.
- Can you strum and do changes without looking at your guitar?
- Can you strum an a patter, do timely changes and in the meantime whatch the footage taken in the cockpit of a train? (example: https://www.youtube.com/watch?v=v0vrhCqFUFE )
- Have you tried 16th note strumming and are your changes still solid without losing track of the drum beat or metronome?
yes? Than you can be confident you’re ready to start singing.
If the things above make you stutter, miss chords or lose timing too much, you need to reinforce those fundaments a little more
To help you with that, this post could help you as well; it’s about getting that rhythm playing to solid automatism so you have more “processor power” to allocate to the singing part.