Nick's description works on my 1987 furnace installation. I remove the wooden front cover at floor height and the on/off valve is difficult to reach but, is at floor level right on the front of the furnace inline with the gas line.
My manual also has the same start,safety procedure. That procedure is only if you are having trouble starting the furnace and it has run for some time without lighting.
My normal procedure is as follows -
(1) turn on gas at tank
(2) light oven burner to be sure gas is flowing (nearest applicance)
(3) move thermostat lever until I hear a pronounced click that indicates the circuit is energized. lever at extreme left is off, should incur some resistance as you move the lever to the right. move until I hear the click.
(4) furnace will take a half minute or so to start blower.
(5) furnace will run for a half minute or so before ignition. ( I can actually hear mine trying to ignite)
My furnace need a new electronic board. If it sits for a long time like over the winter, it gets damp and will not fire in the spring. I put a elec heater in front and blow warm air on the furnace until it dries it out. Works fine for the rest of the year. Also have a catalytic heater.
I will buy a new furnace before I put a board in an old furnace.
I would recommend buying a new furnace rather that letting some repairman get you for $250 for a board and installation. same applies to sail switch or gas control valve. Good money chasing bad.
Learned this the hard way.
Gas control valves stick shut.