For ball/patch combo, try .530 ball and .015 patch. That generally gets you close in a .54.
Clean well as suggested and then get a few hundred rounds through the gun prior to making any major adjustments. I normally start a gun of that size with about 60 grs BP and then work up. Round balls are very forgiving about twist rate so you should see some accuracy potential right away.
Soap and water will do a fine job of cleaning. Add some vinegar if you like, it helps break down the potassium carbonate fouling.
Check the patches after you fire the rifle, they should look like they could almost be reused.
Practice, practice, practice.