basically, at our company, but procedures may differ by company, several critical routes are identified. In the dispatch fase you have to determine if the weight at the critical part of the route is low enough so that you would be have (on top of my head) a 1.2% climb rate at the route MORA. If the weight does not allow this, you need a driftdown procedure. (You simply go in the chart in the FCOM and enter with the MORA and it gives you a max weight.)
The driftdown charts we use just let you insert a point in the FMS and we draw a circle around that point (usually 75 miles) where that circle intersects your route you have to have a certain altitude (usually 25000ft, not FL250 so you have to apply corrections). Before you come to the circle and you get an engine failure you have to turn back, after the circle you continue but in both cases your driftdown procedure would clear all obstacles. the reason for the circle is that it would also apply to direct routings(grid mora).
The actual driftdown procedure: select the altitude you want to descent to. Select the driftdown speed(at this point derived from the FMS) select LVL CHG and let the A/C decelarate to this speed and it will start a descent.
Very short summary, there is a lot more to it (several pages as a matter of fact) but in a nutshell.
Hope this helps a little