Hi Matty123.

The best way is to take off from one airport and fly to another one not too far away, about 100-150nm away, closer if you can.
1. Make sure you KNOW the ILS Frequency and Heading of the runway you want to land on (use the Map in Flight Sim X to find out about the airport information) and enter those in the ILS Tuner on the Radio Stack Panel (SHIFT+8). ILS Tuner is the third one down and has a large white button in the middle, with TEST written above it and ILS written below it.
2. You will need to use VERTICAL SPEED SWITCH, HEADING HOLD KNOB (right mouse click so that it follows the changes as you make them)  and probably ALTITUDE HOLD SWITCH of the A/P for your flight.
3. Once you get near to you destination airport and the ILS "Comes Alive" you will need to use the LOC followed by the APP buttons on the A/P to capture the ILS and Glide Slope for the landing. You should get LAND3 light (green) as your AUTOLAND STATUS and the 757 will follow it in and land itself.
4. Don't have full tanks. If the flight distances is under 500nm use about 20-25% fuel in all three tanks. This should be far more than you need on a 100-150nm flight. The range of the 757-200 is about 3500nm, so use a calculator to figure out how much fuel you will need and add 10% (it's better to be safe).

You will get lots of warning messages on your in the EICAS Message Field because you won't be using the FMC, just ignore them.

I hope this helps and I wish you good luck with it. Feel free to ask for more help if you need it.

Mark

And if I were going longer?

Thanks I'll start short than long!