I'm also a singer/musician and while most of my singing and playing simultaneous experience stems from piano, the idea behind it is all the same.
If you can sing, then singing is generally easy. Therefore, it should be the last thing you add to a song. Learn the lyrics and the melody, and get it to a point when you can sing it without thinking about what comes next. When you can do that, work out the instrumental and perfect it. I don't mean get it to the point where you can get through it most of the time, I mean get it to a point where you can play it perfectly without even thinking about what you're doing. Generally, if you need to think about which frets you need to be hitting for your next chord or riff, you're not ready to start singing. It's not until you hit that point where you can do it without thinking about your next step that you're ready to begin singing.
When you've achieved this, simply start playing and singing. Don't think about anything except what it's supposed to sound like together, because if you focus on either part, then you'll psyche yourself out and lose your rhythm and you'll find yourself either singing the guitar part by accident, or singing in time with the guitar, which generally isn't how songs go.
Just to rehash, if you feel like you can't do either part without focusing on what you're doing, you're not ready to try it together. Be sure of both parts so that you can focus on neither of them.
