You could use your guitar to start you off in the right key,stop playing after a few lines,then check back periodically with the guitar to see if you have wandered off pitch. I daresay you'll get better as you practice and train your ear. Confidence and your ear are key....see what I did there,Martin?
I did a cover a while ago of a Vicki song and enjoyed the process,but I'll readily admit I used a guitar to keep me honest. It's a great exercise and fun to do.
https://soundcloud.com/skub1955/i-need-room-vicki-coverI'd love to play a part in an accapella group,but I have my doubts about being up to scratch. It's a wonderful thing when done right.
Good luck anyway Martin,any vocal exercise is good for ya!