You shouldn't start singing the song until you know the guitar part subconsciously. How do you know it's subconcious? A simple trick is can you play the guitar part without any hesitations or mistakes whilst:
- Reading a book out loud.
- Holding a conversation with someone (being able to listen and respond without pausing).
To learn the guitar part use a timer and perform short bursts of practice. Set the timer for 2 minutes and play the song through. If there's one chord change that you're finding difficult then don't just play the whole song again, focus on the problematic part and play it repeatedly for 1 to 2 minutes. This may seem dull, but it DOES produce results. If you don't believe me, try it out for a few days.
Depending on your level of experience it could take you anywhere up to a month of regular practice 5-6 days a week to learn the guitar part subconsciously. The good news is that every time you practice your song, those particular chord changes will be getting faster, so the next song you play will feel easier if it has the same chords.
Video 4 - Learning the words and using a capo