I have been drinking so this is stream of consciousness, apologies in advance.
If you have usaa, use their car buying service.
If you do not, use truecar to get printouts and offers from them first. Bring paper- physical paper. That makes it real. Have your own financing squared away but do not tell them that. Get their price offer printout during negotiations, then mention you have your own financing later. When asked about the rate (they will) lie and say 1.9-2.9% or so, and make them work to obtain a better rate. Write things down. Waste their time, then leave because you are uncomfortable being pressured. Chances are someone will come running after you to try to get you to sign right away, because they have done no business while you were wasting their time. Dress nicely but not stuffy, and keep things professional. You aren’t there to make a friend, and being unfriendly feels “wrong” but your relationship with this person is transactional, like a prostitute. You each have an objective and ideally land in the middle.
Do not be rushed. Rushed decisions are rarely good decisions.
Take notes while talking to them, occasionally reference your notes, and if they say something that doesn’t match what they said earlier - ask them to clarify which statement is true. Negotiations are like a fight - the first one to backpedal gives up ground and will probably lose. You don’t *have* to buy the car. Remember that. You can walk away. They will try to get you excited, get you chomping at the bit, then they’ll spin the numbers for a low monthly payment for 72 months and you’ll be screwed. Do your own math. Write things down - people who write things down during negotiations aren’t easy marks and they’ll likely try to get you out of there as quickly as possible.
Call and ask what color ST’s they have available. When you go in to buy, you had your heart set on whatever color they don’t have, and you’ll expect a little something to accept something other than your preferred color.
Go test drive a gti, a veloster n, and a few other cars. Get brochures and have them tucked into the back of the little notebook you take to shop for the car. Don’t reference them, but make sure they can be seen. Show you have options.