Many things I discovered through trial and error. However, some things I was taught. My wife, as a present, bought me a community pallet planter making course. Run through the community programme Cultivate Oxford I have used this as the basic template for all planters since.
It was a fun day out, even although we got soaked through due to freezing rain. It was a good example of how easy and versatile pallets are as a resource for making things. I would not say you need this course, if you follow my how to guide on pulling apart pallets and building planters you will have all you need to follow, but it is good to support community groups and nice to meet like-minded people working on similar cool upcycled projects as yourself.