How do I get better at functional programming?

In the craphole I came from they did not teach us any FP at college. So now I see that in Western schools, they teach CS students SML, OCaml, Haskell, Curry, F# etc! I have my mind set on writing an SML compiler using GCC as backend, but tbqh, how can I implement a functional language if I suck at the concept? I have tried to learn but I get discouraged because the whole concept seems foreign to me! I have read several SML books, I think all that there is, I have emailed Professor Appel several times and I pretend to know but I don't know

So, how to get better at func prog? What applications do your recommend I implement to hone my skill? I had half a mind to implement a UNIX shell in SML using only Basis, so the interpreters like PolyC and ML# could run it too — however, I kinda got lost at the start!

If you could provide me with pointers on getting better at functional programming, that would be super duper. Thanks.