Where do I begin diagnostics? on 1992 Ford Taurus

failure to shift into overdrive, then other gears. Once the car downshifts, it just tacs out and I have to put in park and turn off. When I turn it back on if the transmission is cold it works more often, as it heats up it gets harder and harder shifting. (automatic stationwagon)

Asked by for the 1992 Ford Taurus
seek a free diag and est from AAmco and then shop prices