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: http://sheldonbrown.com/wheelbuild.html
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