Fairly simple operation. I use a pure lead round ball over greater than bore size. For a .30 bore I use a .36 ball, for a .45 bore I use a .50 ball, you get the idea.
I use a leather or plastic mallet and tap it in at the muzzle. Then once started tap it all the way to the breech with a brass or hardwood rod. The excess lead will simply trim off at the muzzle. A bit of case lube in the bore will assist the process.
Be careful when it comes out the breech that you don't damage the slug.
Others will no doubt have other suggestions.