... Then I close off vents, all the way on the bottom ...
Nope.
Couple things to keep in mind:
1) Temp swings happen more slowly than instantaneously. Closing the intake is how you shut down the oven. So, don't. Just reduce the air intake but don't choke it completely unless you don't want to cook.
2) WAIT
3) Check it. Adjust, Wait more.
4) You can adjust temp also with the lid vent but just like with a car, you don't want to "reduce power" buy closing up the exhaust. Bad combustion (literally!) and bad taste on the food from poorly-running charcoal with nowhere for smoke to escape. Just ... normally ... don't.
5) Charcoal placement.
A lid thermometer will register whatever's mostly below it. If charcoal is piled beneath it but empty where food is (for an indirect cook) the lid thermo will give a false reading of "too hot."
***************
-- I'd recommend getting a cheap oven thermometer, placed NOT right on top of the charcoal area and at least 1 inch away from food. Open the lid to check that every once in a while until you get a better sense of what's going on. Very simple, very intuitive!
-- Know that this is an outdoor oven capable of having 2, maybe 3 different heat zones depending on how you set it up. That will affect what a thermometer reads ...
-- Practice with a specific cookup you want to do, search or ask for that here and take note of the pictures posted and such. That will illustrate HOW it was done and hopefully, WHY.
Keep going, you'll get it! And when you do (and you WILL) then shazam!