To use the paper clip code reading method on OBD1 vehicles only (will not work with OBD2) locate your cars on board diagnostics port - usually under the steering wheel, around your left knee. It resembles an old parallel port for computers.
There will be 12 "pins" or holes on this port - a top row of 6 pins and a bottom row of 6 pins. The upper right hand hole/pin is called Pin A, directly to the left of this pin (top row, too) is Pin B. You will need to take a paper clip and firmly stick one end of it into Pin A and then take the other end of the paper clip and stick it into Pin B. Make sure each end is in there tightly. This is called jumping or jumpering Pin A and B.
Now turn your cars ignition to on or acc. but do not start the car. Now, you will need to watch your SES light in the dash and it will flash intermittenly to show you the codes. For OBD1 the codes are 2 digits - look in your cars repair manual or consult a technician with what your specific codes may mean for your particular car.
The SES light will represent the codes as flashes with pauses between. Short pause between numbers in the same code and then a long pause between codes. Most cars will repeat a code three times before moving on to the next code. So, for example code 21 would look like this: flash flash, short pause, flash, long pause, and that would be repeated twice more and then move on to the next code if there is one.
Note that code 12 means your cars diagnostic system is operating correctly, and your SES light should flash code 12 when it first starts the diagnostics readout. If you do not get a code 12 your diagnostics system is not operating properly. If all you get is a code 12 then there is nothing that the diagnostic system has to report to you - good news.
Hope that helps,