Locate all the wires and mark them, there should be at least 5 wires, one for the pilot, one for valve, one for the gipper and a common wire, OK, so that might be 4 wires, ummmm well, then 4 wires should also work except the light on the thermostat won't light up all the time. In which case you may need a flashlight to see it, of course that is only at night or if the power goes off in your house or something, but if that happens the furnace will go off too so you won't need the light. Unless you have an old milivolt system in which case you can still use a flashlight.
Where was I?
Oh yeah, so hook up the wires to the different terminals until it works. Have a beer and relax in the fact you just successfully fixed your furnace as good as any so-called, overpriced, think they are all that, professional.
With the money you saved you can go to Walmart and buy some cool stuff.