I'd start by dropping the pan.

Look in the pan for anything other than fluid. If parts, you need repairs. If silt, you have severely worn friction band or clutch surfaces and will likely need to rebuild. If nothing, it might be the front pump which a transmission shop can check by running pressure tests.

The working pressure can be adjusted with the modulator on the side of the transmission (search "modulator") and will firm up shifts (clockwise) if the pump is putting out enough pressure/volume.

You might get lucky (if the friction surfaces are not too worn) also by rebuilding the valve body (which can be done without removing the transmission from the car).

Most diagnosis starts with fluid levels and condition, pan contents, and pressure tests.

If the car has had a recent "flush" of the transmission fluid, this is a normal way for it to die.

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Gone to the dark side

- Jeff