there is no simple answer,
actially the method is very simple, but the explanation is not - things like that need to be shown
if you are in US go to home depot and get a can of "faux finish glase" or whatever it is called - needs to have word glase in it, made by Behr.
and raw umber tint - pigment dispersion.
1) Put two coats of clear glaze (it will look milky white in the can - will be transparent when dries) - to seal the plaster.
2) slightly tint the glase with it and paint over the whole thing, then take a moist rag and wipe it off, leaving as much as you want in crevises. repeat when dry if desired.
Home Depot should also have it already mixed "antique glaze" as I remember. get that and you dont need to mix anything.
All water base:
check some videos here - just watch the technique - he uses oil paints - you doNOT need to:
utube is your friend
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center