I can play guitar but I am having trouble singing lyrics over the songs. What is the best way to teach myself?

what I do with my piano playing is I just learn the song on the piano until I can play it with out thinking about it because my hands get used to the movements. then I learn the lyrics of by heart. then I put the two together slowly verse by verse. I practise one verse at a time and just link them together. it can take anywhere between 3 weeks to 3 months for me to be able to do that. it depends on the difficulty of the piano. this works for me but it may not work for you. sorry if I didn't really help... just keep on practising and practising and you will soon get there.