Indexing the spark plug

The goal is once you have the spark plug tightened down you want the electrode pointing at the exhaust valve like in the picture to the left.  Usually once you tighten the spark plug you aren't that lucky which is where indexing washers come in handy.  
Each one of these indexing washers you add will adjust the position of the electrode by 1/4 turn.  You just keep adding the washers until you reach the desired position.  The fewer washers you use the better.
It's definitely easier to do this with the cylinder head off the engine.  If you want to index the spark plug with the cylinder head on the engine, start by drawing a line on the spark plug indicating which direction the electrode is pointing.
Now there's a little guess work involved however you can see here we have a really good idea our electrode is pointed toward the exhaust valve.