I was lucky. I already had extensive knowledge of Docker before starting the certification for Kubernetes developer (CKAD), and I have an employer (Ordina) that gives me the space and time to invest in myself.
So I claimed a week of preparation and did the whole Kubernetes for Developers (LFD259) course. To follow this course, you have to prepare a practice environment, and you are given instructions on how to do that on AWS or Google Cloud (which can result in extra costs). It is also very possible to create a cluster on your local machine. To make my life easier (and cheaper) I opted for the last option and created a vagrant set up with instructions for it here.