In my roundhouse area, I just applied it to the edges in order to leave the ties exposed. I laid the ballast on and then poured the glue on using a mixture of 1/2 Woodland Scenics "Scenic Glue" and 1/2 warm water shaken together well in a plastic bottle with a cap and angled nozzle which worked out well as a glue. I believe the bottles are known as "hairdresser's bottles" and are available inexpensively on Amazon.
You need to be careful about the glue you use because if it doesn't contain a "wetting agent" the mixture can just run off the surface of the ballast and not sink down and glue everything together. Scenic Glue contains a "wetting agent".