Solve a problem with Mathematica you already solved without Mathematica.
Read at least one book of Roman Maeder: still very good: Programming in Mathematica
Look into the Standard Add-On Packages Mathematica distributes by default.
Read the Mathematica Journal on the Web.
Do not think you learn mathematics if you learn Mathematica.
Do not think you learn graphics design if you learn Mathematica.
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Even simpler - read the Wolfram blog, it contains cute program snippets.
Try to reimplement a standard built-in command and compare their work with your work ...
Ask questions, do not convert it into a bureaucratic job to know all commands and options in advance - you never will.