How do I build a simple planter box?

It is an easy DYI project. Cut the long boards the length you need and cut the end boards the width you want the box. Nail or screw, screws are better, the smaller boards inside the end. Cut the bottom board large enough to cover the whole bottom being sure to drill in some drainage holes. I line mine with a thick mil garbage bag and then fill with the good potting soil. You can put at least one small hole in the bottom of the plastic. Use wood that is at least 1/2 inch thick. Put in enough screws/nails when attaching so that the box does not have a chance to buckle with the moisture. If it does buckle it will be hard to straighten it out again.

I agree with Joy, rectangles the length you want, with the bottom bigger in length and width to support the all four sides. I would use treated or marine plywood so that the planter won't rot from the weather it will be exposed to, use marine poly to top coat it for extra protection, a number of coats will last a long time. Put in your drainage holes and cover it with the garbage bag inside the planter and make holes in the bag where the drainage holes are in the planter. I then line the bottom and a couple of inches up the sides with landscape fabric to keep the soil from coming out the drainage holes. Fill with your potting soil, put in your plants and enjoy them for a long time to come.

I have a post on ones that we built with legs. You could just build the box and skip attaching the legs. It was fairly simple! https://withinthegrove.com/diy-raised-garden-boxes-with-organic-soil/