Take the A44 towards Oxford, then without entering Oxford, take the A34 going north to Bicester and continue north until the big roundabout for the M40. This is the roundabout you would come off the M40 at, take the A41 north to Bicester.
This brings you to a roundabout (where the A41 goes right to Aylesbury). Go straight on, to a smaller roundabout surrounded by Tesco, a pub, a shopping centre and a petrol station. Drive straight on. At the next mini roundabout, also drive straight on. After two sets of traffic lights you come to another small roundabout, where the road forks ahead to the left and the right, or goes sharp right into the town centre. Take the road forking ahead to the right. Under the railway bridge. Past the petrol station. And another twin mini roundabout appears, where you go straight over both. There is a speed camera on the road infront of you (30 mph), on the traffic island. They turn it round every now and then to monitor each direction. Continue along this road to the large roundabout. Here you take the A4421 to the right (there are other signs saying RAF Bicester, ignore them). This road is called Skimmingdish Lane, and the airfield entrance is on your left about 150 yards along this road.