Could you elaborate a bit? Are you talking about building some new design for a black powder cartridge? Or are you talking about the steps involved in building up the various components to load a black powder cartridge?

I am going to go out on a limb a bit and guess that your talking about putting together a shootable PB round. Also guessing that you will be using a straight (45-70) or straight taper (40-65) case. Basically you want your powder colum to come up to the base of the bullet with no air space. To determine the powder height in your case you will need to know your case length, your bullet length and your overall length of the loaded round. From the LOAL (loaded over all length) subtract the case length  and then the bullet length. The negative number is the depth from the case mouth to the top of the powder. Now adjust your powder measure until it throws a charge that you have poured through a drop tube that comes to approximately the depth that your have determined, a little less depth is desired so that you can compress the powder a little bit. Place a card wad on top of the powder and if your only going to compress a little bit .030 or so you can do that with the bullet. 
For a cheap card wad, you can use a cereal box. I also like to place a piece of wax paper between the bullet base and wad, this will keep the wad from sticking to the bullet. For a drop tube, many reccomend 36", personally I use an 18" drop tube made from a copper supply  line for a sink. For just starting out, you can just slowly pour the powder through a funnel into the case. 
I have attached a pic showing the relationship of the powder, wad and  bullet.
Sam

Preparing a black powder reload round for a single shot rifle:
Case: Anneal mouth of case to 650 F, then
* chamfer the case mouth ID with a 22 degree reamer
* expand the case mouth with a small bell to be able to seat a lead bullet so the case neck tension is 0.001 to 0.002" with the proper size expansion die
Primer: With BP reloads, recommend the Federal GM150 Match primer
Powder charge - caliber dependent:
(You need to Login or Register to view media files and links)
Powder Compression:
the Black Powder Cartridge News -2009 Fall - "Some Black Powder Compression Data" article by Bob Woodfill:
Caliber - 45-70
Lyman 457124
Constant volume of FFg powder used
Compression tested: 0" - 1/8" - 1/4" - 3/8" and 1/2"
Best 3 Shot Groups 100yds ... 5 shot groups were tested also, with the same compression values except for Goex (1/4") and Schuetzen (1/2")
Goex - 0.62" with 1/8" compression
Goex Express - 0.26" with 1/4" compression
Schuetzen - 0.61" with 1/4" compression
Swiss - 0.56" with 1/2" compression
KIK - 0.65" with 0" compression
Wads:
Various wads can be used for BP reloads - cardboard, fiber, felt and low density polyethylene (LDPE)
Bullet:
Alloy - 1:20 or 1:16 bullets that are proven accurate ... Postell, Creedmoor, Lyman 457125 and 457677 Matthews tapered
Seating:
Finger seat the bullet to the proper overall cartridge length then remove the case bell with a full length sizing die.  Do not crimp the case

Be sure to insert a test loaded round in the chamber of your rifle to insure it fits

BUY THIS REFERENCE BOOK:
(You need to Login or Register to view media files and links)