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:

http://www.youtube.com/v/3EsEEcAITqo&hl=en&fs=1

utube is your friend

or here:

http://www.shopthecolorshop.com/store/index.php?main_page=index&cPath=17&sort=20a&page=1