You must be flying on normal auto pilot to do an "autoland"
or an ILS approach.
You must have your Nav 1 and 2 set to the correct frequencies of the airport you want to land at, you must set these either before you take off or when you are at least 50 miles from the landing to give yourself plenty of time to prepare for the landing.
You access your nav 1 and 2 in the radio panel. Give it a try landing on runway 90 left at Heathrow.
The frequencies for Heathrow are 113.60 for the airport VOR, enter this in the active box in Nav 2 and enter 110.30 ILS frequency in the active Nav 1 box.
Position the 737 about 50 miles west of Heathrow at 5000ft,
In the Nav 2 insrtument dial you will see VOR DME, this will give you a pointer to Heathrow and a miles to Heathrow read out.
When you get around 30 miles out start your descent to 3000ft and lower your speed to 200mph then ask for permission to land, at about 28 miles out you should start to pick up the ILS signal, Keep checking the Compass instrument dial below the attitude indicator this will now give a more accurate mileage readout to the landing point on the runway.
When you are approx 18 miles out switch off the auto altitude hold and the heading control switch, switch on the APP switch, your the 737 should now make flight adjustments to follow the ILS signal and land itself.
If you have not done it already put your throttle control to idle as your auto pilot should already be controling your speed.
The auto landing system will not control your speed, flaps or gear you will have to do that.
Switch off all the auto pilot systems as soon as you land or you won't be able to stop or steer on the ground.