When cutting shingles, cut the BACK with a utility knife - not the front - (this will save the blade) - just score the shingle and bend it until it breaks.
Hold the nails between your index finger and middle finger wih your fingernails resting against the shingle/roof (ie hold it like a cigarette) - that way, when you hit your hand instead of the nail, it hurts a lot less.
Start at the bottom corner of the roof.
Put two courses of shingles on the bottom row (the first row) - lay the first course across the roof - then lay another course directly on top of it so that the seams and tabs are staggered. (i.e. on this second course, cut three inches off the first shingle before you lay it down so that all of the shingles in this course are aligned three inches off of the course beneath it.)
Lay the second row of shingles so that they half cover the first row (the first row had two courses - one directly on top of the other. Now we're on the second row) Again, the seams and tabs should be aligned three inches off of the rown beneath it - so, in other words, after you get the first row in place, just alternate cut/uncut shingles on each row ... (e.g. third row-> cut off 3 inches from the first shingle you lay, fourth row-> do not cut the first shingle, fifth row ->cut three inches off the first, etc...)
Do this on both sides of the roof (assuming the roof is just a traditional gable roof) - start at the bottom corner and work up to the top.
At the top, cut off one-third of a shingle (one TAB). (Most roofers will also cut off the top corner of this tab, leaving you with a piece that looks like a barn (sort of) ______ /...... |......| --------
Lay these tabs across the peak of the roof (turned 90 degrees from all the other shingles)