I am sure that "search" will turn up more information than you'd ever care to read...!
In short, this is what I do...
1) Run glue along each side (slope) of the roadbed. I use cork.
2) Put your ballast in place with a spoon/brush/other tool. The glue on the sides of the cork prevents the ballast from getting away.
3) Wet the ballast with alcohol. I use a 50% isopropyl straight from the bottle (it is from the dollar store). I use a kids' medicine dropper to put the alcohol on. I find this works better than trying to spray.
4) Put on the glue/water mix. 50/50 is about right. Because the ballast is already wetted with the alcohol, the glue mix flows right in. I also use the dropper for this.
5) Let dry. Clean tools, including dropper.
6) Scrape away any misplaced ballast with a fine screwdriver or other tool.
Hope that helps.
Andrew