Track ballast is referred to as a layer of crushed rock or gravel upon which a railway track is laid.
I would suggest that you paint the areas of your base board where the track will be laid, using a paint colour similar to that of your scatter. Some people don’t paint the base boards, but in my experience it pays to. If you don’t paint the base board when the ballast gets knocked off then you see the original colour of the base board. The paint helps to make it less visible if the ballast gets disturbed.
Now pin your track down into place, remembering to make sure that its all connected well and that the trains run without derailing or stuttering. It is best to run the layout to make sure all the problems are ironed out prior to you ballasting. The scatter and glue can stop the points from moving or working correctly. It is best to tape over these sensitive items.
The next step is to sprinkle some of the scatter on to your track and with a small paint brush or a piece of card, move the scatter in between all the sleepers to a level just below the top of the sleepers. The card is also useful for creating a straight line of scatter about 1cm away from the track to form the edge of the track bed.
Once the scatter is where you want it the next step is to secure it in place. To do this you need to make up a mix of 50% PVA glue and 50% water. Experienced modellers also add just a touch of washing up liquid to the mix. This helps the mix to flow easier. The next stage is to spray the scatter lightly with water and simply apply the mixture to the scatter and you should see it soak in. Some people recommend a syringe or dropper for applying this mixture. The scatter has to be left for 24 hours to allow it to set, remember the PVA mix will try clear.
The last stage is to run a track cleaner over the track to remove any glue and clean any loose scatter away. At our club we usually end up with some scatter glued to the track or the sleepers, so we just use a small screwdriver to remove it (gentle does it!). Make sure that the inside edges of the track is clear of scatter as it will cause your trains to derail.