Grading a note is very similar to grading a coin but there is also the necessity to "feel" the note.
When looking at a note similar to a coin you're going to need to scan all areas (the fields) but pay particular attention to the corners and edges as this is the area that will show signs of damage first.
place the note inbetween your index finger and thumb to feel the weight and crispness of the note.
While lightly holding the note between the index finger and thumb pull the note out of your grasp with the other hand but feel for the amount of embossing the not still has on both sides.
Place the note up to a back light or window and look for level 1 security features in the note as well as any other signs of folds or creases.
Look at the note from the edge so it makes like a fine "horizon line" and look at the quality of the note if it looks like it has been pressed or treated.
Smell the note for any chemical treatments if it's suspect
Finally give one last look at the note (I usually start in the upper left corner) as if the note has 4 quadrants and go over each section noting any conditional issues.
With that you can probably learn to grade 65 and below with some practice...