Take the 5 gallon bucket. You have to cut off the bottom from it leaving a hollow cylinder. Use a jig saw for it or any other tool that is available and you are comfortable with. Take special care of your hands and prevent them from any possible injury during this part.
Now dig up a hole in the garden, greater than the diameter of the bucket. The hole should be greater than 12 inches. Place the bucket into this hole. The rim of the bucket should be at least an inches above the ground level. The rest of it should be in the hole. Firm up the soil around the cylinder so that it is solidly placed in the hole. Fill the hole with soil and pack it.
Now fill the bucket with potting soil that you are going to use to grow mint. The whole idea is to create a barrier around the mint population. Mint has runners that propagate underground and spread like wild fire. The bucket inside the soil is going to contain the mint runners and prevent them from spreading. Mint grows to a maximum of 12 inches, so the bucket is enough for it.
Plant the mint seeds in the bucket. Grow them as routine plantation. When you want to harvest, snip off the tops. The mint is going to keep getting fuller and bushier but will stay inside the bucket. You can safely grow it alongside other herbs.