Our Corten Steel Planters are made to rust and will start showing the signs of rusting if you just put them outside for a couple of weeks and let Mother Nature take its course.
If you do not want to wait for a couple of weeks, wash the planter with warm water and soap when you first receive it. This will remove any remnants of oil and the water will react with the metal initiating oxidation (rusting). A periodic mist of water will accelerate the oxidation process, especially if you are in a dry climate.
Spraying vinegar onto the planter will give it the rust look in a matter of minutes. This rust however washes off, so the next time it rains, your rust will be gone. The planter really just needs a few months, with or without vinegar, to gain a natural layer of rust and seal.