Start out slowly playing chromatic scales. Play each note at each fret up and down each string, singing each note aloud as you play it. This will help you start to develop vocal melody while keeping your fingers, thumb or picking hand busy. The chromatic exercise serves as a good warm up and, after you gained some proficiency at it, move on to playing both the major and minor scales up and down the neck (remember to go slowly). Sing out the familiar “do-re-mi,” etc. This helps you get used to singing while your fretting hand keeps active. Once you feel you’ve gotten a grasp of these basic exercises, it is time to move on to playing songs. Choose one song you have been tasked at playing and singing or pick one you are interested in and play only the roots notes of the chord or the first note of each measure. Again, sing out the lyrics while you are playing. At this point, you are concerned more with being able to get the words out and not miss any roots. After you feel this is becoming second nature, structure your practice session around being able to play more of the song and simply speak the lyrics. Run through this method a few times, then move on to tackling playing as much of the song as you can while singing lightly to yourself, this time being concerned about not missing notes in the vocal or the bass line. Once the song becomes more natural, it is time to crank the volume on both your playing and singing. Go through the track at home. Try it out at your band’s next practice.