Turn the bike upside down and stand over the cranks looking down the length of the rear wheel. Do up the right hand nut (tight-ish) and leave the left-hand nut loose. Now push the wheel to the left and tighten the nut on the left. Loosen the nut on the right and push the wheel to the right. Tighten the right-hand nut. Loosen the left-hand nut and push the wheel to the left. Tighten the left-hand nut... Repeat until you've got the required tension.
Turn the bike upside down and stand over the cranks looking down the length of the rear wheel. Do up the right hand nut (tight-ish) and leave the left-hand nut loose. Now push the wheel to the left and tighten the nut on the left. Loosen the nut on the right and push the wheel to the right. Tighten the right-hand nut. Loosen the left-hand nut and push the wheel to the left. Tighten the left-hand nut... Repeat until you've got the required tension.