Apart from the RULES of Deck building, here's advice on how to do it well from a competitive player: * Try never to exceed 43 cards, keep as close to 40 as possible * Stick to 1 or 2 main themes, only use 2 if they work together. Sticking a load of strong monsters in a Deck won't get you very far. * No Normal Monsters without a VERY good reason. * Around 20-23 monsters, 12-16 Spells and very few traps. Most traps should be staples (Mirror Force, Solemn Judgement, Bottomless Trap Hole, Dimensional Prison, Trap Hole, Royal Oppression, Torrential Tribute, Dark Bribe and Skill Drain are considered among the few usable trap cards) * Very few level 5 or above monsters in the main Deck unless they have an EASY TO USE effect that Summons them from your hand - even a Monarch Deck,