In addition, if the DEP/ARR page lists transitions for the selected STAR and/or runway, be sure you pick the correct one. Next check the RTE pages and resolve any discontinuities (if your last route waypoint matches up with the STAR there should be none; I usually decide on a preferred STAR before I leave and make sure the rest of my flightplan matches that, but if the controller tells you something different you may get a discontinuity. Discontinuities can be resolved by deleting the appropriate lines on the RTE page, and if that doesn't work, try deleting it on the LEGS page). Don't forget to press EXEC.
Finally, go to the LEGS page and make sure the altitudes match up with those on the approach chart. They usually do but it still doesn't hurt to check. On the FMC, 1000 means 1000 exactly, 1000A means 1000 or above and 1000B means 1000 or below. You can enter an altitude without entering a speed.
To fly the approach correctly, go to the INIT REF page (it should default to the APPROACH subpage while in flight) and select your desired approach Vref speed and flap setting, it lists two options one for 25 and one for 30 degree flaps, you can select one of those (do this relatively close to the airport because the FMC will adjust the Vref speeds based on the expected landing weight, so if you need to hold a long time or something or take a detour after you entered this setting it could end up incorrect; I usually do it when starting the STAR). Now once more go to the LEGS page. You'll notice it has you set for Vref + 5 (approx.) at the runway fix, but still 240 at the fix before that. That's not a very comfortable way to land. Adjust the speeds so that you have that Vref + 5 minimum speed at the final approach fix (usually the point where you line up with the runway and/or intercept the localizer), and adjust the speeds at the fixes before that so you can slow down in time if necessary. To enter a speed without entering an altitude on a LEGS page, type it as "155/" (without the quotes). If the altitude on that particular line is in small font (i.e. not set but calculated) you must enter an altitude or it won't accept your input.
Now if all goes well the FMC will slow you down right up to the final approach fix (don't be afraid to use spoilers if the FMC tells you "drag required"). You should be completely configured (flaps set to the setting you selected on the INIT REF page, gear down, autobrakes set, spoilers armed) by that point. You can fly in LNAV/VNAV mode right up to the localizer intercept. At that point, assuming this is an ILS landing, press the APP button on the MCP (you should do this while below the ILS glide slope). All three AP buttons should light up on the MCP. At some point near the runway the autopilot annunciation on the PFD should change to LAND 3 indicating that all three autoflight systems are linked to perform autoland (if any of them is malfunctioning you would see e.g. LAND 2). When approaching minimums the vertical and horizontal autopilot annunciation should include the text FLARE and ROLLOUT. You are now set for CATIII autoland (note: FS lets you do this even at airports that don't have a CATIII ILS, if you want to be more realistic check beforehand and don't use autoland but switch to manual when crossing decision altitude (or earlier)). The autopilot will take you down, flare, and track runway centerline on rollout while you can go have a cup of coffee or something
. All you have to do is engage reverse thrust when you touch down.