I haven't done a Halloween costume more than a handful of times in my life but when I do, I prefer to do it right. I didn't go for Total Historical Accuracy or anything – you can, for example, get authentic 1850s pants and a shirt, but they're expensive so I went with basic modern equivalents in the correct color and close enough style.
Step 1. Grow a giant beard (Step 0.5 is "Be Eastern European, or possibly Italian.")
Step 2. Carve said beard into sideburns down to the jawline and a handlebar mustache
Step 3. Brown cotton or twill pants
Step 4. Natural henley-type shirt. Cut or rip off the elastic at the wrist. Open the neck.
Step 5. Either make leather knife holsters for a belt or, if you're basically talentless like me, use leather shoelaces to hang butcher-type items from a belt.
Step 6. Be realistic about the fact that no matter how cool it might look, you probably can't walk around outside or in any kind of business establishment brandishing a real meat cleaver and knife.
Step 7. Make a red/white/blue sash either by dyeing a white cotton strip or sewing together colored fabrics
Step 8. Make or buy the appropriate cap. This was the hardest thing to find. I eventually bought an aviator style cap ("Snoopy cap"). Either cut off the chin strap or pin them up inside the lining of the cap.
Step 9. Tie a leather strip around the cap at the brow. This keeps it in place and tight against the head.
Step 10. Use tall brown leather work-type boots, pant legs tucked in. If you're willing to spend insane amounts of money you can get vintage knee-high types.
Step 11. The vest. I had a lot of problems with the vest. Ultimately I bought one, although given time and a sewing machine I think the best option would be to make one to fit your torso. As it was, I safety pinned the vest to eliminate some of the billow and extra material.
Step 12. Ask the Christian Lord to guide your hand against Roman popery. Yell at people a lot.