OK I've been doing this for 40 years so I have some experience with it. On lead vocals learn the melody of the song and get it firmly in your head listening to the recording you want to emulate dozens of times until you have it down cold. Your bass won't help you at all with a melody. Words can be read off a cue card but unless you can sight read a vocal music score you better learn the melody with you ears. Next simplify your bass lines and memorize that pattern you'll use to a point where you can do it almost unconsciously. Either the bass playing or the vocalizing have to become automatic and I've always found it's easier to automate the bass. You'll need most of your brain to concentrate on recalling they lyrics and phrasing the vocals. Practice on your own as much as possible and more importanly with the guitarist or keyboard player in your group so that you have some chordal and/or melodic accompaniment to work off of. Use a metronome or a tick track. Bassists tend to lose their perception of timing when they sing. Don't push and don't pull. Create a pocket you can sing within and lock into it. The nice thing is that you can control the tempo and the space needed to fit your vocals with your playing. Don't over do either component to begin with. Get it 80% of the way you want it and play it into shape from there. As you get more comfortable you can venture into more complexity. Straight Rock, Country, and Blues are the easiest genre to sing over bass. R&B, Soul and Funk the hardest because of the synchopation. Pick your material well. Backup vocals require much the same but are somewhat easier if you have any kind of ear for harmony at all. When taking a harmony line, subject to your vocal range, take one higher than the lead vocal line since that will allow you to hear yourself over the low frequencies shoved at you by your rig. The high tenor parts in country rock are usually handled by bassists. Think Randy Meisner and Timmy Schmidt of The Eagles. Get the band together to work on harmonies at a break down rehearsal. Just one guitar, bass and keyboards is you have them. Go unplugged, no percussion because it's a whole lot easier to here the vocals blend that way before you go to a balls to wall rehersal. Just start out with stuff that's easy to do like blues or 4/4 rock or country. It takes a while to seperate your brain into two working parts but the less strain you put on one half of the challenge the more you'll have left over for the other. Good Luck!