Here are some tips that can help you sing and play guitar at the same time:

1. Start with a simple song that has a steady rhythm and a familiar melody.
2. Work the guitar part into muscle memory by practicing it until you can play it without thinking.
3. Use a metronome to improve your timing sense and keep the beat.
4. Practice counting the beat aloud or humming the vocal melody as you play guitar.
5. Make sure you’ve memorized the lyrics of the song and sing them in a comfortable key and vocal range.
6. Take vocal lessons or watch YouTube videos to improve your singing skills.
7. Record yourself singing without playing the guitar and listen for any mistakes.
8. Replace the humming or counting with actual lyrics and practice until you can sing and play smoothly.
9. Practice, practice, practice!

Here is a 9 step method of learning to play the guitar and sing a song at the same time :
1. Choose an easy song that you love - This means that you should start out with a song that has simple chords and a simple melody. Go for a song that you love listening to, as you'll already know the lyrics and rhythm.
2. Start combining your voice and the guitar - There are 2 ways to learn to perform a song's guitar and vocals at the same time. It is your choice which you pick, some people prefer one over the other:
- Strum + hum - This method involves focusing on the guitar part first. You play the chords you already know, and hum the melody to the vocals.
- Play + sing - This method involves focusing on singing first. You sing along with the recording of the song, then try to add in your guitar playing.
3. Practice strumming patterns - Once you have mastered playing chords while humming or singing along, it's time to start practicing strumming patterns.
4. Practice chord changes - Trying to remember how to finger a B7 chord while playing is going to make singing at the same time virtually impossible. Your guitar playing must be at a level where chord changes are effortless.
5. Practice playing without looking at your hands - Once you have mastered chord changes, it's time to start practicing playing without looking at your hands.
6. Practice singing without playing - Once you have mastered playing without looking at your hands, it's time to start practicing singing without playing.
7. Combine singing with playing - Once you have mastered all of these steps individually, it's time to start combining them together.
8. Record yourself - Record yourself playing and singing so that you can listen back and identify areas where you need improvement.
9. Keep practicing! - The more you practice, the easier it will become.

I hope this helps! Let me know if there is anything else I can help with!