Edit: Oh yes, I should read before I reply. Not how, but how often. 20-25k.

Orig post:
4-1-gallon jugs marked in 1 qt increments
12 quarts synthetic fluid or your favorite brand
2-people that can count to 12. I use my 10 fingers and 12 toes.
1-clear hose
Dump fluid from drain hole in pan, reinstall drain screw and refill. Remove upper rad line, insert hose. Start engine while output person counts the qts coming out. Input person tries to keep up but funnels will not allow it. Turn off engine if output exceeds input by 1-qt. Run to clear. There will be about 4-5 qts left. Cycle gear shift, about 2 seconds per shift. Maybe 2-3 quarts remaining by this time. Finish a little low in fluid. It is easier to add fluid then to drain. Check fluid but don’t over fill cold. Check fluid level a few times after driving about an hour.

A few caveats:
If the fluid flush cycle has been abused, drop the pan leaving the filler tube attached and clean it out first.
If the fluid flush cycle had been abused pre flush with the Walmart brand and drive for 2k miles.

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Tom F. Three Bricks, maintain two more.