In brief, here's how a normal driftdown procedure is accomplished:
A gross ceiling (single engine) based on actual weight, ISA temperature deviation and ENG Bleed demand, is calculated, through the table below: the gross ceiling is then transformed into a net ceiling by entering another table with weight and ISA:
Fuel and time to alternate calculations are made by mean of other tables, then a descent is started down to the EO CRZ ALT, that is kept till the new calculated TOD for alternate/dest airport is reached:
Here's, in a very brief form, a typical driftdown profile for Airbus ...
Happy Landings,
fredgrav