Instructions
Remove the appropriate amount of hitpoints. Characters gain more HP with each level-up. When you lose a level, you will have to remove the HP you gained at said level-up from your total HP. If you have a character with a total HP of 40, and you gained five HP last level, you would have to re-work your total so that you only have 35 total HP upon level-drain.
Remove any feats you gained last level. Every D&D character gains a feat every so often, usually at each third level. If you gained a feat last level, you'll have to remove it when you apply the negative level.
Remove any spells or learned powers.
Remove any attack bonus and saving throw modifiers.
Remove any ability score increases. This one hurts the most. Your character gains a new ability point to spend on any of his six primary stats at every fourth level. You will need to remove this point---if applicable---every time you apply negative levels.