Well if you're 120 miles out and cleared for the approach that gives you more than enough time to plan a normal descent from FL200.
Most of the time, the answer is that you will plan a roughly 3 degree descent so that you cross the IAF for the approach at the minimum published altitude, complying with any intermediate changes in the MEA or published altitudes on the approach plate. Say the minimum altitude for the IAF is 10,000 ft....subtract that from the altitude you're at (20,000-10,000 = 10), multiply that number by three (30), then add 10 miles to allow yourself to slow down. In Mexico they want you at 250 or less within 30NM of any airport below 10,000 AGL, and at 200 or less within 5NM below 3,000 AGL. So about 40 miles from the IAF, you'll want to start down if you need to cross it at 10,000 ft.
This assumes that the approach has some kind of transition from the airway structure to the approach....most approaches in Mexico use DME arcs to serve this purpose. If you don't have a transition, then you just fly the MEA to the VOR, hold until you bleed off enough altitude to commence the approach.
Transition altitude vs. Transition level....
Transition altitude is the point on climbout where you will set you're altimeter to ICAO standard (29.92) and began reporting altitudes as Flight Levels.
Transition level is the point on descent where you will set you're altimeter to the local setting (QNH), to accurately read the aircraft's altitude above mean sea level.