Suck old fluid from res.
Disconnect res return line.
Route res ret line into waste can.
No need to start engine.
With wheels off ground, work steering wheel back and forth. The old D-ATF gushes out into waste can.
Refill the res with new D-ATF periodically while doing this.
When you have exchanged a sufficient amount of D-ATF, re-attach return line to reservoir and top off res.
Start/run engine. Stop engine. Check level of D-ATF on dipstick and adjust. If it foams up(usually accompanied by a odd whine sound), let it settle for a while and add D-ATF to replace the settled-out air.
Detaching the reservoir in order to clean the screen at the bottom is optional, but will need to be done at some point.
Use only Dexron ATF.