[HELP] how to add game types

[sniper]

ok so me and my friend got an idea for a game type but we r not sure where to start. it involves something like tony's sniper fort in a way. we need help in
1 where the hell to begin
2 how to put permeant structures like the sniper fort into a map
3 how to put it in a mod menu under gametypes

if you don't know about the fort here you go
tony's sniper fort