So the DME is a Distance Measuring Equipment that is radio based, so you would need to tune a radio to the assigned frequency, and the your instruments will show the distance from that Device (DME).
You can obtain the frequency from the chart for the approach they have cleared you for. That way as you approach/cross the four mile range from that device you are triggered to do something as instructed.
Let’s make this easier. If you have set up your NAV1 for the ILS approach, NAV1 should be tuned to the frequency for the ILS. It should already tell you the distance that you currently are from the station there the ILS is located. That distance is the DME. The closer you get to that station, the lower the distance (DME) will get.
What @1656497 needs to do is watch NAV1, and maintain the assigned speed until NAV1 hits 4DME, then manage his own speed as needed for landing. This involves actually looking at the instruments instead of letting the instruments handle the entire critical phase of flight.
I concur. There’s a difference between knowing how to use the autopilot, and actually understanding what the aircraft is doing as a whole.
As said, maintain 160 knots until the DME reads 4.0 NM. When you see 4 miles, that’s the end of the speed restriction. At this point, throttle back and slow down to your final approach speed for landing.
Hopefully that’s helpful but if nothing else, don’t put that thing down at 160 knots.