Fill a large plastic container with salt and vinegar. Place the metal planter inside, so it's submerged on one side. Let sit for at least an hour (longer is totally fine, too). Turn planter over and repeat soaking on other side.
½ Cup Salt, 1 Gallon Vinegar, 1 Metal Planter
Pour out vinegar/salt mix, keeping container. Place planter back in container and pour hydrogen peroxide over the planter. Allow it to soak for an hour on each side. After it's finished soaking, remove planter and rinse out inside.
1 Bottle Hydrogen Peroxide
How to Assemble the Planter
Place rocks in the bottom of the now rustic planter.
1 Cup River Rocks or other stones
Then determine which order herbs will be arranged and begin planting them atop the rocks in the planter.
3 Herb Plants
Use any extra soil from the herbs' original cups to fill in any gaps in the planter. (You can also use additional potting soil if needed.)
Soil
Lightly water your new herbs and put your planter on display!