This is the method that we have used on Heworth Sidings and we have found that it has worked well for us.
Our method for applying the ballast is:
Mask of areas of the board you don't want to get wet
Plug any holes for dropper wires etc with cork underlay or other substitute
Sprinkle the ballast onto the tracks and work with a brush until you have a profile and coverage your happy with, if you have problems getting any stray bits of ballast from the tops of the sleepers, give the rail heads a tap with the back of a spoon. The vibrations cause the ballast to jump off the sleepers.
Wet the ballast with a Water (75%) and Surgical Spirit (25%) mix, applied with a small pump misting spray bottle
Apply glue (PVA Water mix with about the same consistency as milk) with a syringe, running the needle down the inside of the rails releasing the glue as you go
Wait an hour or two, then apply more glue to make sure everything sticks well.
Here are some pics of the process:
Masking off the area to be worked on
Masking off the rail ends to keep the ballast on the board and in place
Holes for the dropper wires
Holes for the dropper wires with cork underlay filling
Dry ballast applied
Ballast after wetting
Ballast after glueing, some touch ups needed where the dropper wires are
I also took a time lapse video of this work being done on one of the modules