Lacing is generally regarded as a very complex science. Actually it is not so difficult if you follow exactly the steps explained for instance here:
However, it is easy to make mistakes when trying to follow these methods.
Here I explain a no-think lacing method which is really easy. In the example I change the hub of a wheel, but the same method may be used to copy the lacing pattern of an existing wheel when building a new one.
Spokes, rim, hub, as needed. You will provably need new spokes, which must be calculated for the hub and rim dimensions. Happily, spokes are cheap in internet and can be easily calculated online, for instance here: http://www.bikeschool.com/tools/spoke-length-calculator
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Put paper tape over the rim and hub.
Number the spoke holes of the rim. If you want to build another wheel, give the number one to the hole next to the valve opening.
Mark the side of the freewheel.
Follow the spokes and note the same numbers in the hub. Indicate with an arrow if the spoke is inserted from the outside or from the inside of the flange.
Dismount the spokes.
Cover with the paper tape the new hub and copy the numbers and arrows of the old hub. Pay attention, the holes of one side are at middle distance of the holes of the other side. To get it right, insert a spoke in one hole to begin numbering the second flange correctly.
If you are building a new wheel, copy the numbers to the new rim. Pay attention to begin numbering next to the valve opening. The spoke holes in the rim are shifted to the side of the hub flange. Some rims may begin with the first hole shifted to right or to the left. If the new rim is not beginning towards the same side begin with the number one a hole further. The idea is to let the valve in a place with enough space to use a pump.
Insert all the spokes that enter the flange holes from the exterior side flanges.
Place the hub in the center of the rim. The freewheel side of the hub must be on the side marked as freewheel on the rim.
Enter the spokes in the rim holes having the same number. Screw the nipples, not tight, only some turns.
Twist the hub. Now you can see all the spokes going in one clockwise/counter-clockwise direction.
Take a spoke, insert it from the interior side of the flange and guide it to the corresponding number on the rim. This is a cross three pattern (the most common), you should therefore not forget to go under the spoke of the third (or last) cross before inserting the spoke in the rim hole. You can see that on the figure. You may need to bend a bit the spoke, don't worry about that.
Repeat the operation with each of the remaining spokes. Your lacing pattern-will be finished.
Screw all nipples until you feel that they are equally tight (not too much). Then you should true the wheel. If you don't have a truing stand, you may do it directly on your bicycle frame. If you have rim brakes, you may use them. Just tight the spokes opposite to the side were the rim is stopped by the brake. If you do not have rim brakes, fixing a rule as in the photo will do. Of course, a truing stand is better, but these methods are good enough for me.
Finished. I hope it may help somebody not trusting himself with lacing.