There seem to be lots of different theories about the best thing to use for the track-bed (if anything at all).
My first inclination was to lay the track straight onto the baseboard, however noise can be an issue with this. Some people still swear by cork, others will say it's useless. Being a high-tech sort of person I settled on a modern closed-cell foam track-bed from Woodland Scenics. This comes on a roll with a pre-marked split up the middle.
As I'd only marked the centre line of the layout the split is very useful - I can just split the roll and lay each half up to my marked lines. One thing I did notice though is that the split is not very central. For me this isn't an issue - this isn't a modern high-speed line so the track and track-bed will tend to wander a bit. My aim is that the split in the bed follows the line, and the track follows the split, so the track will be in the correct place if not necessarily central to the bed.
Most of the advice suggests Copydex is as good as anything for sticking the bed down, so a pot was duly purchased. It's then just a simple matter of gluing, waiting, sticking, weighting and waiting!
I knew those Harry Potter books would come in useful! First section is weighted down after gluing.
I decided to treat the surfaces as impervious, the foam certainly is and the baseboard has been painted, so that meant putting a thin layer of glue on both the waiting until it's touch dry. I decided to do relatively short lengths each time to keep it easier to work and let each one dry a bit before continuing. It's probably not the quickest way to work, but I can do other things in the mean time.
Glued down section - the pin is there to mark where the points will start.
The track-bed I'm using has sloping edges to make it easier to achieve a realistic "shoulder" when ballasted. The whole thing works out that by cutting the shoulder off you can infill a double track easily, with a bit of cutting around the points.
Gluing the infill on the main station section.
Any excess of glue around the bed can easily be peeled off but it's important to cut it first otherwise you tend to pull the glue from under the bed itself - leaving it to thoroughly cure also helps. I found a disc-cutter ideal for this (in the same way as a pizza wheel cuts through the topping).
Completion of the first bit.