The swash leveler tools linked to by midntpumkin are the most practical to use. Keep in mind that leveling the swash is important throughout the pitch range. You'll find that leveling the swash at zero pitch doesn't necessarily give you leveled swash at full negative or full positive pitch. You'll have to use servo travel adjust (or end points) to make the swash level through out the pitch range. This way you'll prevent unnecessary drift as you apply more pitch/throttle whether in normal or inverted flight.