I have a lot of ELL students. Our district allows any and all modifications, except for modifications to state tests (since that's all mandated by the state).
I am NOT an expert, but I have worked hard to try to modify things to help my students. One thing is to be very careful of the phrasing you use for a question. I try to avoid using colloquial words and phrases, slang, or sentences that are too long or have unnecessarily difficult phrasing. I avoid a lot of those questions that are "designed to trick the student" (those seem so common on state assessments though). I teach vocabulary using very short, simplified definitions. I also try to provide lots of translations so they're seeing the words in their own language and in English. For assessments, I send my ELL students to take the test with the ESOL teacher or a bilingual aide, who helps them understand even better. I know that option isn't available to everyone; I've been very thankful for that opportunity.
I know reaching my ELL students is likely my biggest weakness. My goal is to engage in a lot more professional development to improve in this area.