Get yourself to the south side of the Drum Tower. If you're taking the subway, get out at Exit C of Shichahai Station on line 8, turn right and walk up. You'll see the tower in front of you: it's a big square red building, past a fork of traffic.
Alternatively, take a taxi to "Gulou" and you should end up at the same spot. Either way, walk over the fork of traffic and get right up close to the barred gate.
Note this is not the entrance to the Drum Tower (the way in is on the north side, next to a quieter courtyard), but is where the walk begins.