Parktools have some good illustrated instructions for most things.
The 2 varieties of stem (threadless/aheadset and quill/threaded) have different methods First ID your stem type.

The vertical bolt for qill stems is about 5-6" long. If you have removed it, put it back gently and thread back in. When you have about 1/4-1/2" protruding, place a wooden block on top and strike gently with a hammer to force the wedge bolt down. The stem can corrode and sieze in place. Hitting more wont release it. Jiggle the bars side to side to get some free rotation. You may need a lot of force on the bars. Once you get a little rotation, jiggle more and upwards and it comes free.
To release a siezed quill , Sheldon Brown site has some tips on the correct solvent.