One of the things I've had to do on the road towards Ruby competency has been to write my own versions of array methods from scratch. I ended up reusing them quite often before I realized that the classes I was making could be subsumed into Array or Hash or whatnot as required...but at least I got lots of experience.
In order to write these, I had to think from a very different perspective: one of creation moreso than use. I had to think in terms of future use of this code, rather than trying to get it to produce something for me in the moment. That was a really interesting and important conceptual switchover for me, and I hope I keep it, because it feels more rigorous.
It was a huge struggle to conceptualize how these methods worked under the hood, and I had to strengthen both my knowledge of Procs and self in order to do it.
Writing out my_each was the biggest hurdle; after that, I could
piggy-back off of it to write my_map, my_select, and my_inject methods.
Next hurdle will be to write out all the different kinds of sorting algorithms. Computer Science 101, but for me, it will probably be a revelation.