1) Buy "Professional guide to Wheel Building" by Roger Musson, it is going to be the best 9GBP you have spent in a long long while. [HINT1]
2) Read it, twice!
3) Buy the rims and hubs before you buy the spokes (and get the necessary tools too if you haven't already).
4) Measure the ERD (Effective Rim Diameter) using the old-spokes with glued-on nipple method that Roger describes [HINT2].
5) Buy the spokes that spocalc.xls then calculates for you.
5) Lace your wheel like Roger describes, to the letter.
6) Tension your wheel like Roger describes, to the letter [HINT3].
HINT1: Do not read any other sources, Gerd Schraner's book is just pure nostalgia and does not help you much. Especially his explanation for tensioning your spokes should be ignored: while it might get you a straight wheel, your spokes might have wildly varying tension, and are therefor likely to either break due to fatigue or have the wheel go out of true quickly.
HINT2: For creating the cut-off spokes for measuring the ERD as Musson describes; screw your nipples onto your spokes so that your spoke only _just_ comes out of the nipple into the groove for the nipple-driver. This is the measuring length you should use. If you use the absolute top of the nipple for measuring the length, then you will have no room for error, and you will very likely use up all of the thread on the spoke while bringing the wheel up to full tension (this is the experience bit right here). If it is still inside the nipple, then you most likely will end up with too short a spokes, with thread still showing, this too is a nightmare for wheel-building (your nipple-driver will not disengage). Once you bring your wheel up to its final tension, the spoke (especially double butted spokes) will come slightly further out of the nipple as with the measurement-spokes.
HINT3: For the final stage of tensioning, where the spokes tend to turn with the spoke-key, I marked the rim-sides of the spokes with different colour alcohol markers. This gave me the ability to view the turning of the spokes, and to undo it, close to the rim and nipple, without hampering the spoke-key. Since this is an alcohol based marker on stainless steel, you can rub it off afterwards, or you could just take some alcohol to wipe it off. I just kept it on now, knowing full well that most of it will disappear soon enough in the rain and mud.
I am using Extreme Airline 3s, which i got from Rose. These are rather deep rims that are very stable and sturdy, and they have a wear-indicator still. The joint is not done well, and you will always have a third or so of a mm difference in diameter there, but for trekking or mtb tires, this is no issue, it is just annoying when working on the wheels in the stand. Because these rims are so sturdy, the Schraner method becomes quite unreliable, you can much more easily get away with differently tensioned spokes, as the rim is much more likely to even differences out for you instead of showing where the differences are. You actually need to pluck the spokes instead, like Musson describes, early on in the tensioning process, to get rid of the differences in tone and therefor tension.
I ordered a pre-built set of Airline 3s (28" with LX hub and 3N72 dynamo) from Rose more than a year ago, and they seem quite sturdy and have served me well so far. But, sadly, these pre-built wheels were not up to tension, which I could hear on steep climbs as the spokes were rubbing against eachother with heavy and changing load. They were subsequently very hard to tension further, my guess is because of badly oiled nipples before assembly.
Recently I built a first set according to the Schraner method, and while this went well, and the wheels feel good, I am not sure how good they really are as I haven't used them yet. It could be that they go out of true quickly, especially after pumping quite a bit of heat in them going down some slope in the Fränkische Schweiss. The spoke lengths I used for the 28" Extreme Airline 3 rims, triple crossed (of course!) and with 12mm nipples are 276mm for the front, 281/283 for the back.
For the 26" version of the same rim, with the same hubs, same lacing, same nipples, I used 246mm for the front, and 251/253 for the back. This was calculated with spocalc after measuring the rim, according to Musson, and the ERD is 523mm (after correcting for my mistake). These wheels are for a velotraum cross crmo frame that I am just now building up, so there are no kms on them either, but I have a very very good feeling about them, as I did use Mussons book for them, and the wheels came together as good or even better than described. So while my own handiwork is still untested in real-life conditions, at least I can tell the difference between Schraner and Musson, and the spoke lengths are (now) correct too ;)
In any case, if you are into cycling, have done all other jobs around the bicycle, and mastered them, already, then try your hand at wheel-building to complete the skill-set. It is not black magic, it is actually highly logical, but you should not use sentences like "How hard can it be?" or "Right, I'll get my hammer!" when doing so. Read the right book, get the right tools and the correct spokes, and then take your time; it is really very rewarding.