I think this is a really good question, a few things that helped me improve my testing skills are:
I developed a simple API using express to expose a database and access it through postman. It helped me in understanding what is an API actually and what am I actually testing when I say API testing. I also developed a simple API service for providing API ...
Generally speaking, using scientific method on anything, i.e. trying to understand things and to create hypotesis that will confirm or disprove a theory about any one of the above-mentioned (but not limited to) categories.