From my experience with another game, I can only say: Don't!
"Approving" of guilds quickly leads down the slippery slope of being accused of playing favourites. It may not matter much now, but once there is anything implemented, that enforces any kind of competition, envy, greed and jealousy are likely to show their ugly heads. Remember: Social status alone can be something to fight for (and being "approved" can mean a lot in terms of status).
Also it will be very likely, that you waste a lot of time negotiating with guild about approval status. There will be those who barely meet the criterias and others who barely miss them, but feel that they are not doing any worse then the former (imaging going to kindergarden, asking the children to draw something and then reward the best pictures with candy).
Last but not least: Guilds change with time, as they loose and gain members. If you want to approve of guilds, you'll also have to think about revoking approval (back to the kindergarden metapher: Try taking candy from a child).