Put the piston at TDC. Take the hose of the air compressor with the proper end on it (that fits the spark plug hole) and an air handle. With the valve cover(s) off, verify the piston is at TDC by checking for play on both the intake and exhaust rockers of that cylinder.
You press the air valve of the air compressor hose and pressurize the cylinder. The pressure will keep the valves from droppin' into the cylinder while you have the rockers removed to replace the valve stems.

http://vettenet.org/vlvseals.html

This link is for a Vette small block but also explains how to do it.

 

Sounds like a blown headgasket to me too.

 

what engine is it?, if its a 289/302/351 make sure the head gaskets arent on the wrong sides, ive heard that is a common mistake. so it may not be blown, just on the wrong sides. that may not cause it to burn coolant, but it could be a start to somethin

 

Try doing a pressure test on your cooling system. It will tell you if you have a leak into the engine. To make a lot of white smoke the coolant has to be getting into the combustion chambers it wont get there through the valve seals.