If you're judging as well as hosting then study up on the rules and REG, mostly everything from deck building rules to Withdrawal. In the latter pages you can be more selective in what to study such as Dominants (especially with the change of how they're played), battle resolution, curses/covenants etc; the other stuff, such as what cards are considered Roman or of the Corinth church, are more identifiers that don't really need to be memorized as they can be quickly referenced for verification.
If you are confident on your ruling, and it can be backed up by the REG and/or elder ruling, but one of the players still wants to dispute it stand by your decision and kindly inform them that your ruling stands and the game will continue on. Make sure you show the player as soon as practical your source of what you're basing your ruling on. It doesn't necessarily have to be right when it's disputed but very soon after while you research it. Just make sure you're confident in the rule before the game continues while you're looking up the rule/precedent in the REG and/or forum. You don't want the other player (or both players) to have to wait a long time while you're researching it and run the risk of timing out.
Charge practical entry fees but don't let yourself go into a big deficit covering the cost to pay the application fee and/or products. Do your best to at least break even.
Enjoy, and have fun with it! I've found that hosting can be just as much fun as playing.
