Travelling through a tunnel the first few times can be daunting. Some tunnels are wide enough for two boats to pass, but some are for single way working only. These will be identified in your waterways guidebook.
Go slowly - In the dark it is difficult to steer a straight course and easy to lose your bearings, so go slowly. A useful tip is to switch on some cabin lights - these will light up the tunnel wall and help you judge how close you are.
Oncoming boats - Slow right down when you get near to an oncoming boat to give each other maximum clearance. Make sure your tunnel light is adjusted so it does not dazzle oncoming steerers. You will find you get the best guide to your position by having it shine slightly to the right onto the roof of the tunnel.