There are many possible methods, and which one best suits you will depend to some extent on your longer term intentions. Temporary or alterable methods include using a moulded foam underlay or tacky mat material to simulate ballast. This means that the track layout can be altered and adjusted , and even completely lifted and reused with little difficulty. Then there are more permanent methods, in which crushed stone or scatter materials are glued in place around track. Subsequent alterations to the track layout are more difficult and may require fairly extensive repairs to the scenic treatment, and it may not be possible to salvage all the track in a usable condition if a complete change of layout is required.
I will leave the moulded foam underlay and tacky mat techniques to someone who has actually used them.
Should you decide that glued down ballast is the technique for you, then there is a lot of preparation required ahead of laying the ballast, so that you are not left with a frustrating mess which is really difficult to get working reliably. You need a stable track base, and a track layout which is really reliable in operation, thoroughly tested with the rolling stock types you plan to operate. All the wiring should be in place, and if the points are to be powered then motors or linkages to the chosen drive system have to be in position. Dependent on the style of ballasting you wish to represent, you may want to use the technique of laying the track on a strip of shaped cork, so that the track stands above ground level, enabling the 'shoulders' of the ballast formation to be represented. And a final tip, it is usual to paint the rail sides if going to this much trouble over ballasting: plain track can be done very quickly after ballasting, but the rail sides on points are much more easily painted before they are laid.
As for ballasting techniques, just put 'ballasting' into the search option, top right of the header bar.
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