It's really no different than teaching it to "non-struggling students," except that you'll need more patience! Just be consistent with showing them letters and telling them which one it is. Eventually, they'll remember! It just may take longer, and that's fine. If you need to simplify things so that they can get answer correctly and gain some self-confidence, write your "letter of the week" on a markerboard along with another letter. Ask them which of the two letters (say our letter is "T") is T. Then pair T with another letter option, and again ask them which one is T. Keep rotating out the "wrong" letter so they get more chances at correctly identifying. This method helps a lot in the beginning!