How do you indicate the rhyme scheme for different stanzas in a poem?

This is done by assigning a letter to each line within a stanza (group of lines), where lines having the same letter designation would rhyme within the stanza. Examples could be ABAB or AABB or ABCABC.

ABAB would indicate that the 1st and 3rd lines rhyme, as do the 2nd and 4th.

AABB would indicate that the 1st and 2nd lines rhyme, and the 3rd and 4th lines rhyme.

Various forms of poetry have their own scheme, for example a limerick would have the rhyme scheme AABBA.
You name the first rhyming Word "a" and then move to later letters. If a Word rhymes with a Word from an earlier line, we use that earlier line's letter. For example, a rhyme scheme might look like this: aabba.