you fix it with work top clamps underneath and biscuits, the silicone is also a fix but is used more to prevent water from getting in the gap to the ends of the worktop.
if the room you are planning on doing it in is below the attic then u should go into the attic and see where the ceiling joists are and screw to them, but if they run in the opposite direction the you will have to put noggins between the ceiling joists.
or if u havnt fitted the worktop yet or you have alot overhangin on each end, you can fit end panels, i use them on all kitchensthen u only have plinths showing at the front.
normally there is a plastic (like maths paper with all squares) jig that comes with the kitchen, but i never use them anyway i just find the centre of the draw and mark it by hand
