I'm not sure what kind of pistol you have, so I'll just tell you how I load my Colt 1860 .44 replica.
1) Measure out 35gr. of Black Powder or equivilant (I use Triple 7) in a powder measure. (35gr. is not a set-in-stone number. That's just what I use in mine. Play around with loadings to find out what works best in your's.)
2) Pour BP from powder measure into cylinder hole.
3) At this point, you can opt to put a wad (such as Ox Yoke Wonder Wads) over the powder. Compress the wad tightly against the powder using the rammer.
NOTE: If you do not use wads, then you will have to put a grease, such as vaseline, crisco, or TC Bore Butter over the ball later. If you go this route, just seat the ball directly over the powder charge, and put a generous amount of grease over each ball in the cylinder. This will help prevent chainfires, which is when a spark will 'jump' to the next cylinder hole and touch off that charge. Not a good thing. The wads will also prevent this, but will do so in a faster, cleaner, easier manner. I highly recommend the wads.
4) Place a ball (likely a size .451 or .454 ball for .44 revolvers) over the cylinder hole. Rotate the ball under the rammer, and ram it down. You only have to press down enough to firmly seat the ball against the powder.
Repeat steps 1-4 for every other hole in the cylinder.
NOTE: If you did not use wads, then at this time, put some grease over each ball. Put enough that it will seal off the charge from possible sparks.
5) Place a percussion cap (My 1860 .44 uses size #10 caps) on each nipple.
6) The gun is now ready to fire.