How Do I Set The Timing For The Lifter Valves In My Lawn Mower?

Well that depends on the make, type, model of the engine. Now if you are looking for valve clearance specs, I need more info.  If you are looking to ask about valve timing, you first disconnect the spark plug wire and ground it.   Take the engine off the base.  Drain the oil.  Remove oil pan.  You will now see the cam and crank gears.  Now grabbing the PTO turn the crank and see if the timing marks on the cam gear and the crank gear match up.  You will probably have to turn it a few times to make sure you go through all the strokes.  If they don;t turn the crankshaft until the timing mark points towards the cam.  Then gently lift the cam just until the gear is clear of the crank gear.  Turn the cam until you can see the marks align  with each other.  Sorry about the explanation.  Its hard to explain but easy to do.