If you go in at the gate on the corner of the South Circular, where it meets Lordship Lane, just before the Church and opposite the Harvester, and walk up the path (Cox's Walk), you can get into the woods that way. There are then two directions you can go in, straight ahead, or turn left and go over a bridge over the old railway track. As others have said, I think there are maps at the entrances to the woods. If - whichever of the two paths you take - you then aim vaguely straight ahead, you will eventually come to an incline which leads up over a rather spooky boarded up entrance to an old railway tunnel. Go up the path up the incline, and at the top is another gate. If you go out of here and turn right (I THINK! CHECK!) you will come to Dulwich Wood House (pub) on the left, which is quite handy. Wear wellies if it's been raining, the paths in the woods can get quite muddy. Have a good time, I love those woods! Edited to say: It's spooky because it's always exceptionally cold in the valley bit where the tunnel entrance is, even though it's boarded up. OOERR!!