what i suggest is first just play some chords and sing the root of the chords, with the rythm of the chord. that shouldnt be too hard.
then try addin notes that are in the chord but not the root. but every time the chord changes sing the root.
dont play to complicated rythms or anythin.
after this try and expand what you can plabit by bit. make the rythm more complex, or the singin but not both at the sames time.
give it a go.