I'm only a bit ahead of you in SU Carb lore. You'll get it just stick with it and don't be afraid to peek inside things.

The screws change the resting position of the throttle plate by moving the cam on the side. By turning them clockwise (down) you open the throttle plate. Screwing them back out out closes the plate.

The position of the two plates needs to be synched and that's what the carb tool you've bought will help you do. Run the engine to normal temp, unhook the throttle linkage and remove the air filters. Put the synchronizer into/over the intake of one carb, adjust the screw until you reach an optimal idle speed, note the vacuum reading on the tool, put the tool into the other carb intake, adjust the throttle until you reach the same vacuum reading.

Adjust each until you reach the idle speed you want.

There's a more to this, but lets just start here and see what you get. If you play with too many things at once you'll never get it all sorted out.