CardsplainThe recipe: 17 lands + 23 spells = 40 cards. Of the 23 spells, about 15–17 should be creatures. The rest are and a few other spells.
Why 17 lands. With 17 lands in 40 cards you hit your land drops on turns 2, 3 and 4 in most games, without drawing too many lands later. With 15 you'll miss land drops and lose with cards stuck in your hand.
Splitting the basics. Count the colored mana symbols on your spells and split your basics roughly the same way. Bilbo's Deadly Slice () needs two black mana, so a deck with several cards like it wants more Swamps: for example 10 Swamps and 7 Mountains. Also look at your cheap cards. If all your are red, you need red early. A common dual land for your deck (such as Goblin-town for Goblin Army) makes either color but enters . Play it instead of a basic.
Creature count. Creatures attack, block and carry your . In our simulated drafts, nearly two out of three decks ended up with fewer than 15 creatures. Too few creatures is the most common deck-building mistake. A card that makes a creature (Goblin Plate Mail makes an token, and can make a 1/1 Soldier) counts as about half a creature. If you only have 12, cut your weakest noncreature spells ( first) for creatures, even plain ones.