Over Drive Light Blinks On And Off

[cassj]

1991 LS 400

I watched a mechanic take of the pan of the transmission of another car and relace one of 4 small cylinoids I think it was gold in color nor sure. It took all of 20 minutes and the customer was charged over 300. I have been in a comma for the last 6 months and my budget is low. I am assumin ths is the problem on my car because sometines the yellow over drive light does not blink and the car goes right into snd gear. How do I tell which one I need and can I ddo it myself. Oh and wher can I get those parts

Thankyou