For a first I recommend having the barrel inletted and ramrod hole drilled in a blank, I am not to high on precarves. I only did one precarve without the lock inlet, the precarve was lopsided, it took me a bunch of work to fit my parts on correctly.
For me it is a lot easier to build from a blank with a good full sized plan to follow. I found that drilling all the holes for barrel and ramrod pins as well as lock bolts is much easier with square stock instead of the rounded precarve wood.
Another thing about a blank is you can put in just the right drop and cast off to fit you, with a precarve you are stuck with the drop that is carved in from the factory.
If you do get a precarve, get it with the lock panels shaped but not with the lock inlet. This way you can position the lock perfectly to get the correct touch hole position. With the lock inlet done, if it isn't in the perfect position to line up with the touch hole you have to move the barrel, down or back to position everything correctly.
