The way I create cheat sheets is governed by the following principles:

• All game rules/mechanics/procedures (and sometimes optional rules) must be 100% included.
• All game rules/mechanics/procedures should be as concise, clear and unambiguous as possible.
• Non-essential materials (setting details, GM/player advice, elaborate examples, etc.) are left out.
• GMs and players should be able to entirely run the game with just the cheat sheet.
• Exception: For games with extensive lists of skills, spells, etc. such lists might be left out.