One more thing came to mind: do NOT sing off-key. I can't think of anything more detrimental for voice than singing off-key.
What I mean by that is that if you sing without being assured of the voice, quite tentatively and maybe even wavering a bit, you're bound to be stressing the voice and 'singing too much'.
So start with really simple melodies, or just one note or two, long legatos. Play for instance just open G and A and sing to that just wooo...oooo...or something. Pick two notes you know well and aren't too high.
But BE SURE of the note. Be so sure you can set aside the guitar and wait a while and then be able to belt out the same woo-oo in exact pitch. And when you're sure of the note and it isn't too high, you CAN really belt it out. Imagine you're not singing, you're yelling at some young punk trying to steal your car or something

'Heeey Youu!' but exactly in tune.
Unless you're really constantly producing sound the wrong way, this kind of natural-pitch yelling should happen easily and when you learn the notes and can hit them with exact accuracy and conviction, you could do that all day and not strain your voice.
Only and ONLY when you can do that no problem and make a sound loud enough for neighbors to hear, it's time to go onward to more notes - play simple patterns and find out how far up or low you can go and STILL retain the very same voice level and conviction. The very second you hit a note that's even slightly harder to sing with conviction, fall back. I suggest playing for instance whole notes in 9 notes rotation, start from G first, 9 whole notes up, I usually just sing 'this sounds stupid' divided on those nine, next start the same from G#, then A, and so on. Now if the A riff suddenly gets hard to sing revert back to G# and do that a few more times.
At best I used to be able to sing from dropped D on low E to something like 8th fret on high E string like that and no falsetto yet, but these days I struggle to go so low, something like F is lowest I can sing OK.