Most of it is drawing out your design on the horn with a pencil, you will need some good books to study originals, one of my favorites is Dresslar's The Engraved Powder Horn. Then you cut in your lines over your drawing. You could use various xacto knives or a scribe. This guy makes really nice scribes:
https://www.etsy.com/shop/scrimshawtoolmanSome people will engrave the pattern on but that is a good bit more of a complicated process, also many originals were just cut in with a pocket knife so you could try that as well.
Then you will need to darken in your lines. You could use pencil lead, or an ultra fine tip sharpie, or use a toothpick to apply india ink. Just dip the toothpick a little into your jar and wipe the excess off on the rim.
The 0000 steel wool is used to buff the color off the high spots. The high spots can also be cleaned off with alcohol prep pads.
You could then finish it with johnson's paste wax or renaissance wax if you wanted to.
An optivisor is a generally useful tool.
I would also recommend you check out Mark Elliot's excellent explanation for polychrome horns here:
https://americanlongrifles.org/forum/index.php?topic=69291.0