The trick with felt is to get it as straight as possible and not have any creases in it.  Pull it tight enough to accomplish this, but again, be careful not to tear it. 
Next up, nail off the first course.  You´ll probably have to do this from the ´high side´ which means you have to either bend over, or (my preferred method) sit down near the top edge of the paper and nail it off.  You must be very careful about moving the paper before it is nailed.  This first course is a bit tricky.  After this, it´s a lot easier.
First, go place a button cap towards the middle of the top edge, again, stay down about 4 inches from the top.  Look for a rafter to nail into.  It´s easiest to find a rafter by looking for a seam where two 4x8 sheets meet.  Nail right into the rafter.  Now keep splitting the difference between the top edge buttons so that you have the paper fairly secure and so it can´t sag any more. 
 

The Nailing Pattern

The pattern I typically follow is like the 5 pattern on dice.  One in each corner and one in the middle.  You´ll sit down at one end of the course.  Let´s say you start at the end you rolled off the roof and cut, since that´s where you´ll probably be anyhow.  You sit down and rest your feet on the felt, being careful not to stress it too much (you´ll know how much is too much by the time you´re done putting your last course on).  You already have 3 buttons on your left so look for your first rafter after that (remember, look for sheets of underlayment joining).  Your felt should have some rows of lines that denote an area to place the buttons.  Like two white lines about 3 inches apart or so, put one button inline with the rafter, and in between those two lines, and the second should be inline with the rafter and in between the other set of two lines.  Now find the next rafter and repeat, nailing two buttons.  Then put one button dead center of those four buttons.
If the distance from the edge buttons to the first rafter is more than 16", place a button in the center of those sets too.  The button that you put in the center won´t be hitting a rafter, so the roof is a little bouncy there, you´ll get use to it.
Continue down the roof, scooting as you go.  If your helper does the same thing (only starting on his right) he´ll meet you in the middle and the patterns should match up.
All that´s left is to nail off the bottom edge.  Put your button caps about 8" apart and just above the drip edge.  Remember that whenever possible, don´t pierce the metal.  The metal expands and contracts differently than the other materials, and the fewer holes you have in it, the better.