How do you write my commander regarding toxic leadership in my unit?

Why write a letter....own it, go to him/her address your issues face to face and if he doesn't get it, you have many other options to influence change. Next higher Cdr, IG, etc.

SPC (Join to see), I see that you mention "a Commander" so are you writing it outside of your COC?

I would caution you to become fully informed as SPC(P) Micah Lavigne stated and advised.

There is nothing wrong with bringing things to the attention of the uppers; they need that from time to time. I once wrote my Company Commander a list of things that I thought could be improved upon in the Unit and he actually took the time to discuss every point with me. Of course some were validated and others were not.

If you truly believe that there is an issue then it needs to be addressed. At a minimum they can move you outside of the situation.

My advice would be to try every level of your chain of command prior to getting to your commander. If not, you are very likely to perpetuate the toxicity and focus it upon yourself.