Assuming you're planting from onion 'sets' rather than seeds, don't plant too deeply. Only the root on the end of the onion needs to go in the ground. Jake's advice about the wood ash needs a little explaining... onions like the acid/alkaline levels to be near 6.0 to 7.0 which is basically neutral. Acid being 0 and alkaline being 13 on a scale. Adding only wood ash is somewhat akin to 0-1-3 (NPK) fertilizer for your plants. Which means it has a very mild supply of phosphorous and potassium in it, but no nitrogen.
Since onions like nitrogen, a good 5-10-5 mix would be best.
Fertilizer shouldn't come into contact with the plants, however. First, form a small trench about 6 inches deep, spread the fertilizer evenly in the trench and cover that with at least 2-3 inches of soil. Then, plant your onion sets 3 to 4 inches apart (depending on varietal, some bigger varieties require even further spacing) but at least 4 to 6 inches away from the aforementioned trench. This is not nearly as difficult as I make it sound...
Water sparingly... maybe an inch a week, unless you get good rain (I'm in the drier part of the U.S. btw). When the plants begin to grow, observe the green tops (lots of sun needed) and watch for signs of 'bolting' where the greens quickly begin to form seed pods... not a good thing. If that happens, just step on them to break them over, but keep watch on them for further bolting.
Grow them as large as you'd like, harvesting just before a forecast frost. We tie ours together and let them dry hanging on the sunny side of the barn...