I've found questions here at ELU related to the meaning of the expression (I'm clear on that), but this is about the best way to punctuate the expression in the title. Possibilities:

a. It's almost ready. I just need to go through it once more to dot the "i"s and cross the "t"s.

b. It's almost ready. I just need to go through it once more to dot the i's and cross the t's.

c. It's almost ready. I just need to go through it once more to dot the is and cross the ts.

a. shows what I would naturally do.

b. shows what appears to be most common (used in posts on this site as well as in Wikipedia), but it using that apostrophe for plural sticks in my craw. I did find some support for this, here, but this also suggests using the apostrophe for letter grades (see c. below).

c. seems terribly confusing, although it follows the convention of pluralizing letter grades (which are not so confusing because they are capitalized).

So... which is it?