For the purpose of this article I will assume that you have laid the track, are happy with the design and you have painted and weathered the track.
You will need some or all of the following tools and equipment:
Bag of ballast
Eye type dropper or syringe
PVA type glue
Fine mist spray bottle
Assortment of small paint brushes or old toothbrushes
Many modellers consider ballasting a chore, but one that is necessary to get the right look. Ballast for railway modellers comes in many sizes and colours. You will need to choose the size dependant on the scale of your model railway, and the colour because of what was used at the location of your railway. Size is really important as over or under size ballast just looks wrong. Colour is not as important as variations occurred around the country dependant on what local stone was available. It is possible to combine two colours to create the effect you require, but mixing the materials must be done accurately to allow you to combine the two colours again should you require more material.
Once you have chosen the material you are going to use for ballasting its time to start laying the material. Ballasting can take a long time, but if done correctly will last for years and will enhance any model railway layout. Badly laid ballast will spoil any layout, and will be hard to remove once applied.
Along either side of your track, draw a line using a pencil to define the edge of the ballast. Dont worry if the line is not straight, as you will be bringing your other scenic material up to this ballast edge. Look at photos of your desired location to find how wide the ballast bed should be. A roughly blended edge will look more natural that a perfectly defined edge.
Sprinkle on a layer of your chosen ballast and carefully arrange it between the sleepers, using the brushes, and up to the edge line until you are happy with the results. I found that an old toothbrush was ideal for this in that it was hard enough to move the ballast but not too hard as to dislodge it. In 4mm scale it also fitted between the tracks. On points or crossing make sure that the ballast does not interfere with the operation of the points. Carefully press the ballast down between the sleepers. It is easier to do short lengths at a time rather than do a whole long length in one attempt.
Once you are happy with the look and position of the ballast you need to
carefully and lightly spray the ballast with water to which you have added a
small drop of washing up liquid. This addition breaks down the surface tension
of the water and allows it to flow better and soak into the ballast. Do not
water log the ballast and be careful not to wash the small granules away.
Once wet, apply a 50-50mix of water and PVA adhesive, again with a small drop of washing up liquid using the eye dropper to the ballast. You should be able to watch the adhesive mix flow and soak into the ballast. Allow to soak in. Capillary action will draw the glue through the ballast so be patient and give it time to work its way in. Again do not add too much. Gauging how much to add will come with experience. If you do not add enough the ballast will crumble and not stick. If you add too much it will wash away. You can always add more where required.
Now you have to leave your big soggy mess to dry thoroughly. This can take some time, and should not be rushed. Do not run any locos on the track as the track power or any DCC signal may be affected by the water in the ballast.
Once everything is dry, you should be able to remove any excess ballast and to add more if required to places where it has not set properly.
Finally, the ballast can be weathered to achieve truly lifelike results. For the best weathering effects, mix together a fine wash of mid-brown/grey acrylic colour and apply, followed by a dry brush with white for that finishing touch. The ballast will look better once the surrounding scenery is brought up to meet the ballast.
Once all ballasting is completed test the track again as you will have small grains of ballast where they should not be, and will have glue on your track. Small bits of ballast can be carefully removed with a scalpel blade, but take care. PVA glue on your track can also be removed carefully using a knife blade. The glue does not really stick to the metal track, but will to areas that have been weathered and painted. Be extra vigilant around point blades for small bits of ballast and glue as these will affect the good running of your model.