Actually, it is quite easy. Simply install the inner spokes for one side and thread the nipples maybe half way. Inner spokes are the ones that pass through the outside face of the flange and end up with the length of the spoke on the inside of the flange. Rotate the hub against the rim to take up all the slack out of those spokes. Install the inner spokes for the other flange, and orient them to produce the same, but opposite angle as the other side. You will observe that the holes for the spokes on opposite flanges are not directly across from one another. Of the two holes directly opposite any installed inner spoke, the correct hole will be the one that results in an overlap of the two spoke as viewed along the length of the axle. In other words, the two spokes on each side of the assembly will cross each other almost where they install in the hub when viewed from the side, or overhead if you have the assembly laying flat on a table. It should be evident where everything goes by looking at the rim - the holes should "aim" at the origin of the spoke at the hub. You can verify this by holding the nipple in the rim without the spoke. There are invariably tell-tale marks on the hub that will show where the spokes originally were located, but if they are insufficient, or you are starting with a new hub, the previous explanation will put you in the right direction. With all the inner spokes installed, you can verify that all is well by simply spinning all the nipples down until the spoke is close to flush with the outside of the nipple. the assembly should be obviously getting tight, and the rim should be looking reasonably straight and true. Back all the nipples off leaving only 2 or 3 threads engaged, and lace up all the outer spokes. It will be obvious where they go. Only engage a few threads on all spokes until all the spokes are installed. You will need the slack to offset the hub from center to engage the last few spokes. Usually ALL the spokes can be installed without any bending or forcing or scratching the rim.
The above is true for 99% of motorcycle wheels. You no doubt will know if you have an unusual pattern.
If you did not coat all the threads with anti-seize compound, get a beer and take every nipple off one at a time and re-install with the anti-seize!!!
Spin all the nipples up about 3/4 of the way. Then creep up REALLY slowly, like a half turn each nipple working your way around the wheel. It pays to have a dedicated starting point, like the valve stem hole or a piece of tape so you don't lose your place. After you have done a few wheels you will be able to gauge how far you can thread on the nipples before it adversely influences your work truing it up, but keeping all the spokes very evenly threaded will pay off so creep up on it until you get comfortable with the work. Remember, every single spoke affects the other 35 (39 etc), so by the time you get all the way around the wheel with only a half turn, you have made a substantial change! A little patience will result in a very surprisingly true wheel when all the spokes are snug, making the actual truing a breeze.