easy, use tonic solfa, do ray me etc,a major chord is do me soh,next thing to do start adding sixths do me so plus lah,or ninths do me so plus ray ,but put ray as far away from do as possible,then start leaving out the [me third] part of the chord but still adding sixths and ninths.
next thing is to think about subbing the dominant seven chord,any chord built on the second note of the scale can be used to sub for the dominant seven,so if the dominant 7 chord was g7,use a chord based on dminor.why does this work?
becausethe notes of g7 are gbdf,the notes of d minor are dfa,so they share two notes in common d and f,of course it is more likely to work if the melody notes are d or f,
one chord i find works very well for G7 is a f d f,you are effectively doubling the seventh note of the g7 chord.another chord that can sometimes work for a g7 is the dominant 11,so instead of playing gbdf try gcdf[this could be a sus4 chord too],this is more likely to work when the melody note is not b,but your ears will tell you.