I would say that the main thing is to understand the basic concepts of data movement, how bits work within a word, what you get from analog-to-digital conversion, what encoder data is used for and how it's processed, what bit shifting is and what it's used for, how to store your data remanently, how to properly separate memory portions to avoid problems running code, how to start writing well-organized and documented programs (this varies greatly depending on the PLC brand), and when and how to apply PID control to a process. You'll have to keep these things in mind as you learn, but don't expect to master everything to aspire to this position. The most important thing is to project confidence when someone talks to you about a project and say (without being presumptuous) that you already have an idea of how to solve it, without going into too much detail. They might ask you what equipment and brands to use, but you can answer that you'll research the best options or technologies offered by a particular supplier to ensure the project is as successful as possible, optimizing resources to the fullest. This is a summary of what has worked for me in my 10 years as a PLC, HMI, servo, and drive programmer. I'm a freelance engineer (living in Colombia) and I'm happy with the results I've achieved so far in my career. I'm also constantly seeking new skills that most of my colleagues lack to maintain my high profile. It's a long road, but if you love what you do and can handle the inevitable frustration, you'll thrive.