The preceding answers are (AFAIK) correct in terms of the activation of widely available, public features of FSD -- namely, Traffic Light and Stop Sign Control, plus the Enhanced Autopilot (EAP) features (Navigate on Autopilot, Auto Lane Change, Autopark, Summon, and Smart Summon), if you don't already have EAP.
The big FSD feature, Autosteer on City Streets, however, is currently in a limited beta-test program. As I understand it, people who subscribe, rather than buy, the FSD package are eligible to participate in this program; but you must allow the car to monitor your driving and "pass" this "test" before getting access to this feature. I haven't been following the details too closely recently, but I gather that Tesla has been admitting few new testers recently, so I wouldn't count on getting this feature if you just want to try out FSD for a month; it might take longer than that to be accepted into the program. Perhaps somebody with more recent experience than I, or who's been following discussions of it more closely, can comment on this point.
IMHO, Autosteer on City Streets, in its current state, is not worth getting. Rather than "Full Self-Driving," I believe the "FSD" acronym is better interpreted as "Flaky Student Driver," because that's how the car drives -- erratically and in an unsafe manner. You have to stay very alert so as to prevent the car from doing dangerous things, which it can do quite unexpectedly. (I was recently stopped at a stop light, and the car suddenly started moving forward without warning, for instance.) A caveat: I'm currently on 2022.12.3.20, which provides FSD beta 10.12.2. Tesla is preparing to roll out a new FSD version that they claim is a big improvement, and a few beta testers apparently already have that, but I do not. I'm skeptical that it'll really be a huge improvement; my expectation is that it will be another modest improvement.
If you want to try an FSD subscription to try out the EAP features to evaluate them or for a long road trip, then a subscription might be worthwhile. The publicly-available FSD features are trivial compared to Autosteer on City Streets, but AFAIK there's no EAP subscription, so if you want a subscription, you'll get those features in addition to the EAP features. If you've already bought EAP, then you won't get much value from an FSD subscription.