Basically, you name a requirement for a card, the next guy makes a card that fulfills those requirements. Try not to make the requirements too specific though, a good guideline would be to ask for something with no more than 4 of the following:
1. Card Type (Weapon, Minion or Spell), 2. Class, 3. Tribe, 4. Statline/Mana cost, 5. Specific synergies (i.e. a card that works with Onyxia, etc.)/General synergies (i.e. supports Pirates, etc.), 6. Or has Battlecry/Deathrattle/Inspire/whatever other Keyword.
Like, say someone wanted a 0 Mana 12/12, then I'd make