It is essential to make sure that all the track functions perfectly, because the track positions cannot be easily adjusted after ballasting. A quick visual check will show up any unparallel lengths of track, and a manual test of the points will identify any that don't work.
Ballasting can be rather a tricky task, and takes some patience to get it looking right. The tools needed for the job are: a bag of ballast (I used a granite grey colour, but it does come in different shades), a ruler, a smallish paint-brush, and some glue (I will mention more about this in a moment).
Firstly, I want to mention a little bit about the function of ballasting and about the ballast itself. In real life the ballast on railway tracks is the small pieces of stone which serves the function of actually holding the track sleepers in place. Model ballast is made from very finely ground granite chippings that resembles a fine powder. It is purchased in small bags - one bag was ample for the size of layout I am building. Once it has been laid, the ballast helps to stick the track to the baseboard - so much so that some modellers take out the track pins holding the track in place! This, though, is very much personal preference.
The ballast is laid in between the tracks, and a little is also piled up on either side of the sleepers (on their outside edges) for a realistic effect. There seems to be a variety of techniques for laying it, but the way I used is as follows: firstly sprinkle some ballast direct from the bag along the centre of the track for a short distance. Next, rub a finger down the track to disperse the ballast powder evenly between the sleepers. The paintbrush can also be used to brush surplus powder to the outside edges of the track to form the 'ridges' I mentioned earlier - I found an old ruler useful here to 'scrape' the ballast into position and also to check that I was getting an equal width all along the edge. Once you are happy that the ballast is on relatively smoothly, it is ready to be stuck down. To do this, PVA wood glue is used. Because of its consistency, PVA is too thick to use by itself, so it was mixed with equal parts of water with a squirt or two of washing-up liquid added. The purpose of the last ingredient is its' ability to break down the surface tension of the glue and water and make it flow more easily. Give the whole lot a good shake in a suitable receptacle (I used an old washing-up liquid bottle), and you are ready to start gluing. A good tip to stem the flow of the glue is to put a small piece of matchstick in the top of the washing-up bottle - otherwise you may find that the glue gushes out rather too quickly instead of dribbling out controllably.
Apply the glue all along the track in the places where you have laid the ballast. Do not worry if the track looks flooded with glue, as it will eventually soak into the ballast. One other important point to mention is to be VERY SPARING with the glue around the area of the points - too much applied here will gum the mechanism up completely and you'll be in trouble.
The rest of the ballast is laid in much the same vein. Obviously not all the track needs to be ballasted - most will be hidden by scenery, so I concentrated on the parts that will be on show, namely the front (station) area of the layout. Once the ballast has been applied to your satisfaction, it needs to be left to dry thoroughly (preferably overnight), before it is 'cleaned up'.
Once the ballast is dry, any surplus grains of powder need to be cleaned off - the quickest method is a gentle vacuum with the hoover! The next stage is to 'clean up' the ballast. By this I mean chipping off any small amounts of surplus ballast that have got stuck to the rails, and cleaning the ballast off the sleepers so that they show up better. Use whatever small implements you have to hand for this job - a small screwdriver and an old scriber seem to work well, together with a paintbrush to get rid of the excess chippings. You will also need to go over the layout briefly with the hoover again when you have finished the clean-up operation. I might add that this is a tedious task - the only way to avoid it is to spend more time initially laying the ballast more neatly!
The final stage of ballasting is purely cosmetic, and you can do as much or as little of this as you wish, e.g. painting the ballast, or painting the outside edges of the rails a rust colour. This helps to add to the realism, but personally I have left my track as it is, because I like it that way and am happy with the way it looks. The only thing I will do before starting on the scenery is to rub the rails down gently with some fine emery paper to get rid of any glue - important to remember before running any trains.