This post focuses on the guitar part. Notice the care taken with each step, and the continued emphasis on learning not to interfere with the coordination you have already established.
The process of learning a song is also the process of studying oneself.
Learning the Guitar Part
In monkey position or while singing, spend time working out the guitar part for the song without singing it. Again, the goal is to take time so that you are trying out parts of the music in the context of giving your directions. When you have become familiar with the guitar part, try accompanying yourself while singing the melody, until you are able to play and sing together.
Singing and Accompanying Yourself with Direction
Singing while playing the guitar is complicated because there's a lot to think about. It's easy, at this point, to start playing the song and to stop paying attention to how you are using yourself. Here's an interesting way of paying attention to how you are vocalizing while playing the guitar accompaniment:
1. Standing in front of the table, place your guitar on its side, facing away from you so that, if you go into monkey, you would actually be able to finger the fretboard with your left hand and to strum with your right.
2. Keeping a hand on the guitar so that it doesn't fall, stand and give your directions in preparation for going into monkey.
3. Take yourself into monkey and give your directions, directing your head forward and up, your head and hips to go away from each other to lengthen your back, and your knees to go away from your hips.
4. Place your hands on the table and give your directions, with the added direction of widening across the chest.
5. See if you can now position your left hand on the guitar neck, and your right arm over the body of the guitar so that you can strum or fingerpick. Give your directions.
When you have tried this procedure several times, continue with the following steps:
1. Go into monkey and give your directions.
2. Place your hands on the guitar as before, and play the accompaniment to the first few lines of the song, not forgetting to give your directions.
3. Produce a few whispered "ah's."
4. Continuing to give your directions and play, see if you can now sing a few lines of the song, being sure not to take a breath between phrases, and stopping the moment you tighten or use effort in any way.
The object here is not to perform the song but to place the stimulus to sing in front of you as the basis for studying whether you tighten or interfere in any way.