Usually with spacers on the the top between the spring and the fork cap. I haven't messed with it on showa twin chambers though. On those probably spacers on the outside of the cartridge where the spring flange stops.
with you sitting on the bike very balanced with little foot pressure on the ground your forks should move 3-4 inches from fully extended. If they move 5 inches then add 1 inch spacers to preload them more.