A nifty little primer, compiled by Nate Piekos.
This is probably the biggest mistake seen amongst amateur letterers. An “I” with the crossbars on top and bottom is virtually only used for the personal pronoun, “I.” The only other allowable use of the “crossbar I” is in abbreviations. Any other instance of the letter should just be the vertical stroke version. Although I would debate it, you occasionally see the “crossbar I” used in the first letter of the first word of a sentence, or the first letter of someone’s name.