You need to get to Kronshdadt for beginning. There are the minibuses to Kronshtadt city from metro station "Chernaja Rechka" (30 min for the trip and 20 rubles for one way ticket). You should get out on the crossroad in the end of the Dike, where the road police checkpoint is. From this point your way leads in the south direction, pass by ugly high-rise houses to the bank of Finnish Gulf.
If you are lucky enough and don't fall through the ice or don't be carried away on ice-floe to the open sea or don't be arrested by local police for absent of local registration, you will get to the fort in 20 minutes. So, good luck and have a nice trip...