One of my new students recently asked about singing and playing the guitar at the same time, and noticed that my beginner guitar lessons do not contain any video material on the subject. This is true, I'm sorry to say that I have no singing voice whatsoever, so this subject isn't addressed on the website at all, sorry about that.
I can't say that I will have anything of the sort in the future, since I sound like a seagull with a fever when I try to sing 🙂 I do know a bit about the technique though, and as I've heard from others and having tried myself, it's not the easiest form of making music. You need to be able to totally separate your singing from your guitar playing, a skill which is quite difficult to master. You could maybe compare it to playing the drums, where all 4 of your limbs need to be performing different tasks.
Here is a general guide to singing and playing the guitar simultaneously: