Have you done the VAT refund process before? There are 2 things you would need to do at the airport: 1. (must) visit the customs office to have your filled-out vendor-issued forms stamped, to present the goods, your passport, & your boarding pass, and then 2. (may) visit the refund counter where you get your cash back after customs has processed your paperwork (you can also do this by mail, if pressed for time).
I assume the watch will be on you/your carry-on (vs. your checked luggage)? In that case you have to visit the customs office *behind* emigration/security (so that you are behind customs should you choose to come back in). Per the (kinda useless) FRA website, that office
is located in B (can't imagine there isn't one in A somewhere), and there's also one in T2. The refund counter, run by the private company who will take a huge cut of your refund, should be nearby.
In the case of checked luggage, you'd check in, get the bags tagged, take them back and walk them over to the customs office *before* emigration/security, where they will process the paperwork and take the tagged bags. That is where you could encounter the long lines of people with lots of luggage. With the carry-on process, the lines should be small to non-existent.