Go to a code practice site like hackerrank or oj.leetcode (personal favorite but I suggest using an external compiler for it) and solve problems. Start from the simplest ones and make your way up. Once you solve a problem, go back the following day or week and without using a reference solve it from scratch. After solving a problem look at the solutions provided in the forums. A lot of the time they will be a lot more elegant and contain things you haven't considered or seen before.

This is only half of it. The second half is to choose a mid size project, write yourself something that resembles a spec, and code it up from scratch. It could be some sys util or perhaps a video game (this will require you to learn library linking).

The third half is to read online articles and tutorials while you do the two things above.

The fourth and final half is to get yourself a book once you have a grasp on the language(s) and learn some of their more complicated idioms and pitfalls.

Also as a side note, at this point in time I don't think the "C/C++" conglomerate make much sense any more as modern C++ looks very different from C. Unless you explicitly need C, I would suggest learning C++ first as it's higher level (and also much larger). Though, idk, maybe learning C first is better as it's the foundation for C++... I guess either is fine, just not both simultaneously.