Park the vehicle on a level surface.
Set the parking brake.
With the engine running (or as specified for your vehicle), shift through all gears (P/R/N/D/L as applicable), pausing briefly in each.
Return the shifter to Park (or Neutral if your vehicle specifies).
Let the engine run for the required fluid temperature/time (or wait until the transmission reaches operating temperature, if applicable).
Locate the transmission dipstick (refer to owner’s manual if unsure).
Pull the dipstick, wipe it clean with a lint-free rag.
Reinsert the dipstick fully, then pull it out again.
Check the fluid level against the “HOT” or “COLD” marks (use the correct mark based on current operating condition).
Verify fluid condition: should be a clear-to-red color (or as your manual specifies) and not smell burnt; avoid dark/black or burnt-smelling fluid.
If low, add the correct transmission fluid type through the fill tube using a funnel.
Recheck the level after adding, adjusting in small increments.
If fluid is overfilled, remove excess using the method specified in your service manual.
Reinstall the dipstick securely (or close the fill port if no dipstick).
Record the fluid level and condition and check again after a short drive if instructed by your manual.