If like me you want long curved platforms that are completely one section, you need to mark out the run for the curves.
To do this you need the platforms in a raw state, i.e. in my case the MDF in one lump. Place the track on to the wood and and pin in place the curve you wish to create. Attach a pencil to the carriage and run the carriage over the track, this will draw a curve on to the wood.
Now use a jigsaw to cut the curve. Once finished fix the platforms in place and lay the track through the curve and test with a carriage then a loco, you can refine the edge if need be.
All my platforms are screwed into the base boards for easy removal, just use some scenic models to hide the screws.