Default Need help flashing a ROM

I have a rooted Verizon Galaxy S4 with a T-Mobile SIM card and running on baseband version MI1.

I want to flash a ROM on it that works perfectly with T-Mobile, so I could get full 4g and everything.

What I'm confused about is what I would have to do first. Update to the latest version first then flash a ROM? Or can I just flash a ROM on the current version.

Any help is appreciated, thanks.