Assuming you are coming down the M1, go to Stanmore tube. It is only 5 minutes off the M1. Come off at J4 (A41) and follow the road to the r'about. Turn right onto A410. 100yds straight over next r'about. Stanmore tube is then about 300 yds on your left.
If you come down the A1, Stanmore is still the best bet in my opinion (although Edgware is closer). Follow the A1 under the M25 and keep going until you come to Apex Corner (A41 r'about). Turn right on the A41until you come to the A410 r'about about 1 mile later. Turn left, straight over next r'about, Stanmore on your left.