With cross patterns, you can lace a wheel up a quarter at a time. One side with spoke head on the outside of the flange, then the other side with the spokes dropping in the same way (head out).
Then you drop spokes so the spoke heads are inboard. When doing the crosses, you should do the following:
If n = number of crosses...
Over n-1, under 1.
For 3 cross, you go over 2, under 1.
For 2 cross, over 1, under 1.
For 4 cross, over 3, under 1.
As pointed out before, you need different length spokes for the different crosses (unless you also change hub or rim in a way that allows you to use the same length spoke).
To calculate spoke length, you can use this:
http://www.sheldonbrown.com/rinard/spocalc.htm
(You need Excel and also need to know what you have but don't need to measure much)
If you don't mind measuring (rims are a pain if you don't know what you have), go here:
http://www.bikeschool.com/spokes/
hope this helps,
cdr