After giving your rifle a thorough cleaning as described above, you will need to select the correct ball/patch combo. You want a combination that is tight, but not so tight that you have to hammer it down the bore.
Assuming you are shooting a .50 cal rifle. it should have a twist rate of 1:48 or slower, (1:60 - 1:70 is best). I would start of purchasing some .490 round balls and .010", .015" and .018" (pillow tick) patches. Or if you don't have the $, start out with the .015" patches. If too loose, get some pillow tick (or .495" balls). If too tight, get some .010" patches. As for the powder, start with 70 gr of FFg and work up in 5 gr increments. You should find a sweet spot somewhere between 70 and 90 gr. For best accuracy results you should swab between shots. If using black powder, a spit patch will do using both sides. If you purchase unlubed patches, you can lube them with bore butter or olive oil. (be sure to squeeze out the excess lube).
Any more questions, don't be afraid to ask