I my opinion the best way is to disconnect the automatics and switch the FD off, and fly it manually. Gear down, and as long the speed is high do not try to capture the GP but keep following it 1 dot or 1 1/2 high. Configure as you go. Order the FDs to be re-armed, final heading and missed approach alt to be set. Order the Vref to be changed to flaps 40 and configure to flaps 40 for the extra drag if flaps 30 was planned originally. When fully configured get the GP centered, and if you were not much too high from the beginning everything should be fine by 1000 feet, otherwise of course go-around.
With automatics be very careful as the 737 can bite back when doing this and you may exceed the capabilities of the auto throttle. I would recommend taking the throttle manually at least. Otherwise do as vasalis wrote.
Here is where to be careful: if GP capture occurs, configured with flaps 40, speed still high and throttles in idle, the auto throttle will not spool the engines up quick enough and will happily take the speed below Vref without adding the necessary thrust to get it back.
Best is not to get in this situation by taking the thrust manually and predict for youself when the engines should be spooled up.
If you do let the automatics take you for a ride, do not wait and explore the autopilots capabilities of recovery from approach to stall. With flaps 40 it will take a lot of thrust to get just 5 knots below Vref back - 80-85 % N1. Put it in manually immediately!