basically start with some simple chord progression witha simple rythm and humm over the top of it. like hum (or sing) (dont bother about lyrics) the note of the chord your playin at the same rythm it is played. so if u are playin a g c chords hum those notes. then maybe try and sing some of the other notes in the chord. like hum the like the e in the a chord. etc. then try to mix these two notes together as you play the chord.
next make the rythm you are playing a bit more complex. whilst keepin the rythm of the hummin simple.
next make the rythm simple again and sing a more comfortable rythm.
then try a more complex rythm on both the rythm and vocals. then just improvise a bit sing other notes in the scale of the key u are playin. or actually wateva notes u want as ;ong as it sounds good.
do this for a few chord progressions. and slowly it will become natural for you. the whole trick of this is to be able to make one part of your mind concentrate on one thing and the other part of your brain concentrate on another. basically multitaskin. its a skill once its gained it isnt to hard. and to make it easy you basically start with the rythm and vocals bein same notes same rythm so they are the same and slowly try and make them different.
then it comes naturally.
actually doin this will also improve ur ear and make u bettter at singin to progressions u havent heard before.
