I'll give this a try as I said I would - off to Dublin for a few days culture next week (Heritage Week) so I'll get as far as I ca in bite-sixed portions.
Early in the sixties, a number of singers on the folk scene approached Ewan and said they were not particularly happy with what was happening in the revival and would he consider holding classes - so, not so much a "self appointed Guru", but a respected figure on the scene asked to pass on tips to fellow performers.
Ewan refused the idea that he should teach, agreed to help set up a group of singers and enthusiasts who would work on each other's singing on a self-help basis.
Early members included; John Faulkner, Sandra Kerr, Frankie Armstrong, Brian Pearson Charles Parker, Luke Kelly, Gordon McCulloch, Alasdair Clayre - Bobby and Helen Campbell were early visitors, but not members.   
He agreed to act as chairman and provide a series of exercises that had been developed to assist actors working in Theatre Workshop - these consisted of:
Relaxation exercises based on those used by T.W. - along with a full explanation of the effects of tension on the singing voice.
Voice exercises - basic vowel-sounds (not actual vowels), to enable a singer to find, control and develop their natural voice rather than that imposed by environment and everyday surroundings.
Singing exercises to improve pitch development, accuracy in learning tunes and handling large and small musical intervals , breath control, word articulation....
These comprised two short pieces from Wagner - one to handle large and unusual interval leaps; another to cope with small intervals with accuracy.
A piece from Gilbert and Sullivan to help accuracy of articulation at speed
A piece of Irish 'showpiece' singing (Rocky Road to Dublin) - again for articulation, also breath control (one verse and one chorus sung in one breath).
Similarly, a piece of Scots mouth-music, (Tail Toddle) - sung at speed - two choruses and one verse in one breath.
Also thrown in as an alternative (for the politically inclined!) was Brecht's 'Lament For the Death of a Comrade' - one of the most complicated tunes I have ever tried to sing - I eventually managed it and can still sing it with a degree of accuracy (not in public!).
These formed the bases of the technical work - once (easily) learned (took me less than a week, thanks to the tolerance of my bed-sit neighbor), they stayed with you forever.
One other technique was the handling of tones and 'efforts' (the latter developed from Rudolph Laben's theory of movement, taught by Ewan's then wife, Jean Newlove, in Theatre Workshop and developed for the voice by Ewan) - both need a little more explaining so will deal with them separately.   
MacColl stressed that technique should not be considered an end in itself, but a means to access the entire range of songs available in the folk song repertoire.
He suggested that, at first, they should be done daily (the lot take less than half-an-hour), and eventually, at regular intervals, to keep the voice in shape and to help push its limits.
The voice exercises helped me personally when, after a long interval of not singing, I found my range had reduced considerably and I had lost several of my most rangy songs - took me a month to get them back.
There were other aspects of technique discussed - cupping hand over ear, back-to-front chair, stance...., but those are the basics.
Jim Carroll