Seat the bullet at it's normal depth. To make sure that you have no air-space, measure the depth of the bullet from the crimp groove to the base of the bullet. Your powder needs to end up filling the case to at least the height of the base of the bullet.
You can do this in one of two ways:
1) fill the case with enough volume of powder to go slightly above the level where the base of the bullet will sit, and then seat the bullet. This will slightly compress the powder, which BP likes anyway. You want maybe 1/16" of compression or so.
2) same as above, but use a compression die or dowel to compress the load instead of the bullet.
#2 involves a bit of measuring and trial and error.
For both of those, if you use an over-powder card, be sure to factor the thickness of the over-powder card.
From there you can start to work up the most accurate load by slightly varying the amount of powder you use, which in turn will vary the compression. Always ensuring that there is no air space. If you want a lighter load, then use less powder and fill the space with either a felt wad, corn meal, or a grease cookie. As stated before, don't mix the filler with the powder, layer it on top of the powder.
Also, as stated in the other posts, don't use smokeless lubes. If you do, your barrel will start to lead very quickly. If your bullet doesn't carry much lube (small grease grooves) you may need a lubed felt wad or a grease cookie.
When you do go shoot it, it will be the most fun you've ever had with a single action.
Fighter Pilot, Black Powder Afficionado, and skeptic.