The most common method for adjusting ignition timing is with a timing light. The light is a strobe which triggers from the number one ignition wire. The light is aimed at the harmonic balancer on the front of the engine. You will need to find the timing specification, which should be on an emissions sticker somewhere under the hood. Once you find the specification, examine the balancer for timing marks. It helps to mark the balancer with white paint or white out to help so the correct point. For example if it calls for 6 degrees before top dead center(BTDC) mark that point on the balancer with the white out so you can see it easily when it passes the pointer.
Connect the timing light, carefully route the wires so they don't get caught in the belts or fan. If the engine won't start and idle, have someone turn the ignition while you aim the light and watch the marks location. You will have to loosen the distributor lock nut just enough where you can turn the distributor to set the timing, but the distributor is not so loose that it will not stay where you point it. Once you have the timing set, tighten the locknut and then recheck the timing.
After the timing is set, and you can start the engine, you need to rev up the engine while using the timing light to see that the timing advances. As you rev up the engine the mark should move about 25 or 30 degrees or so. If it doesn't, this could also be your problem. If the timing advance mechanism is not advancing the timing as the engine speeds up, it will cause this backfire that you are experiencing. If that were the case, you would need to find what's wrong with the distributor.
Also, back to the backfire, if you had a vacuum leak somewhere, it can also cause your backfire. If this were the case, the engine would not want to idle either.