When you leave the underground station take exit 2 towards Shoreditch, keep walking down Old Street and once you hit Kingsland Road turn left. You'll see a little lane called Cotton's Gardens on your right-hand side, under the railway bridge.
Turn left out of the Bishopsgate exit and catch the 149 or 242 buses towards Shoreditch; get off at the Waterson Street stop and cross the road into Cotton's Gardens.
Turn right onto Cremer St and then left onto Kingsland Road. Keep walking down Kingsland Road and you'll see Cotton's Gardens on your left-hand side just under the railway bridge.