Recently I have been trying my hand at wheelbuilding (bicycle).
Its a challenge at first but eventually can be mastered!
Two very good descriptions of the technique are:-
Sheldon Brown Wheelbuilding
and also - scanned from a "Bicycling Magazine" book:-

Divide the spokes into 4 equal groups.
Lace up the first group from the top down placing the "keyspoke" immediately CCW from the valve-stem hole. (normal procedure)
Then drop the next group of spokes from above thru the bottom flange one hole immediately CCW in direction from the key spoke(above)
Lace up these spokes to the next adjacent rim hole CCW from each of the first group.
Rotate the hub CCW to tauten-up the spoke pattern.
The important "commonality" here is everything runs CCW (counterclockwise)
NOW turn the wheel over.
Drop in the next group of spokes through the (now) upper flange and lace these through to the rim using the familiar cross-3 pattern (over 2 under 1).
Still working from above drop through the last group of spokes into the lower flange and lace up these spokes (cross-3) into the remaining vacant rim holes.
This procedure enables gravity to assist us, with each "lacing" as spokes will always be hanging down.
Other methods (as described above) you are working against them slipping back through the holes and falling to the floor!
I've found that it's not difficult to lace-up spokes by working around the rim (held on your knees) those from the bottom-most hub flange.

When finished the spoke on each side of the valvestem hole will lead away in opposite directions, which allows easy coupling of the air inflation line.
Also, a well built wheel should have any hub branding (makers logo etc) opposite the valvehole.


Spokes come in various fixed lengths and there is only minimal adjustment range available.
I needed to lace a larger diameter than normal hub (a dyno-hub) into a 26" rim, and a good estimate of spoke length was required, before commencing the job.
There are some good "spoke length calculators" available on the web,
however my simple solution was to make an accurate drawing (scaled down 2:1) of the hub/rim combo, which fitted onto a A4 sheet; and then simply measure the spoke length with a ruler. (then multiply by 2 of course!)