when i built a quarterpipe, to put the coping on, we drilled a hole on the top a little bigger than the head of the screw then a smaller hole on the bottom just bigger than the screw and we just screwed it in like that
countersinking is like if u are putting a screw in wood and u want it to be flush you take a countersinking bit and drill down a little then u put the screw in, hard to explain