This is the part that looks easier than it is. The spokes must all be tight, so that the wheel is strong and perfectly balanced and the rim runs true. The balancing comprises four separate operations: correcting the true, correcting the hop, correcting the dish and correcting the tension. Part of the reason wheelbuilding has always been considered difficult is that adjusting one of these factors affects all the others.
The four operations listed above break down as follows:
The spokes are laced alternately to the left and right sides of the hub. For example, a zone of the rim that is too far to the left can be corrected by tightening the spokes that go to the right. Since every other spoke is connected to opposite sides of the hub, this can be broken down into a series of very small steps, always truing just the part of the rim with the worst bulge.
The rim must be an equal distance from the centre of the hub all the way around. If it isn’t, the brake blocks will be hard to set up and you will kangaroo down the road, your wheel falling apart in no time at all. If a section of the rim hops outward, i.e. is too far from the hub, it can be drawn inwards by tightening two or four spokes centred around the peak of the hop. Tighten the same number of right-hand side spokes as left. Adjusting the hop always throws the wheel slightly out of true, but try to minimize the effect.
It’s easier to correct an outward hop, where a section of the rim is too far from the hub, than it is a flat spot.
The rim must end up centred between the locknuts, so that when you put the wheel into the bicycle frame, the rim runs evenly between fork legs, between seat and chainstays and between swingarms. Tightening all the spokes on the right-hand side will move the entire rim to the right; tightening all the spokes on the left will move the entire rim to the left. The rear wheel has a cassette bolted onto the right-hand side so that the heads of the spokes on that side are closer to the centre of the hub. This means that the right-hand spokes have to be tighter than those on the left, in order to keep the rim central – that’s why they’re more likely to break than those on the left-hand side of the rear hub.
Front disc hubs also have to allow a little bit of extra space on the left-hand side, but the dishing required is minor.
Tension is tricky to get right. You can easily destroy a rim by overcranking the spoke tension until the rim collapses. For the home mechanic, the easiest way to ensure the tension is correct on the wheel you’re building is by comparing with another set of wheels.
Take hold of a pair of almost parallel spokes on a completed, functioning wheel and squeeze them. Then do the same with a pair of almost parallel spokes on a working set. Be sure to compare like with like; front-wheel spokes have lower tension than backs, and the right-hand side of back wheels has higher tension than the left when the dishing is correct.
“You can easily destroy a rim by overcranking the spoke tension”
Dish: these two distances must be the same in width
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Lacing a back wheel”