Six rules, and the first is worth the other five put together: put a pawn in the centre, then bring pieces out to squares where they hit something. Everything below is a way of saying that more slowly.
The six
- Push a centre pawn on move one, e4 or d4. It claims a square, opens a diagonal for a bishop and frees the queen. A game that starts 1.a3 has thrown away the one free move you get.
- Knights before bishops. Nf3 and Nc3 are right in almost any position; whether your bishop wants c4 or e2 depends on where Black puts the pawns, so wait a move and find out.
- Castle by about move ten. On g1 the king has three pawns in front of it and the rook lands on f1, which is where you want it anyway. There is a page on castling kingside or queenside if you are choosing.
- Move each piece once. If your knight goes f3, then g5, then f3 again while Black develops three pieces, you are two moves behind for nothing.
- Leave the queen at home. She is worth nine pawns and every enemy piece can chase her, so an early Qh5 hands Black free developing moves with tempo.
- Connect the rooks. Get the bishop off c1 and the queen off d1 and the rooks defend each other along the first rank. That is the last job of the opening.
8 
7 
2 
a 
b 
c 
d 
e f g h 1 7 2 a c d f g Look at what neither side has done. No pawn has moved on the a-file or the h-file. No knight has moved twice. Neither queen has left d1 or d8. The whole opening is over in five moves because every move did one of the three jobs: centre, development, king safety.
Where the rules bend
They are defaults, not laws. They tell you what to do when you cannot see anything better, which at the start is most of the time.
A knight moves twice when the second move wins something: 1.e4 e5 2.Nf3 and later Nxe5 is not a wasted move, it is a pawn. Wing pawn moves are fine when they have a job, which is why the Najdorf plays 5…a6 to stop a piece landing on b5. And the queen comes out on move two in the Scandinavian, on purpose, because Black has counted the cost and wants the structure.
What does not bend is the king. Every opening principle you break while your king is still on e1 is charged at double rate, and the common opening mistakes page is mostly a list of games that ended for exactly that reason.
If you want to practise the six on real positions rather than read about them, the opening puzzles start you at about move eight, which is precisely where following the rules stops being enough and you have to start calculating.