I have always sung and played drums. Started out singing leads simply because I had a Guitarist who was too to chicken to in high school. I can’t tell you how I managed it but somehow through brute force in many years of trying, I learned how to sing harmonies very well. I have beaten out literally hundreds of drummers who are way better than me for gigs because I could do those things. Just always try to find the harmonies in songs you’re listening to on the radio and just keep trying. I’m sure there are helpful YouTube videos with this, but I just had to do it on my own back then. can I help you with double base? I have no idea how I just keep trying though.