In romaji, the ways that verbs inflect is fairly straightforward to indicate. For example, if you wanted to describe the process under which 五段動詞｛ごだんどうし｝ change from 辞書形｛じしょけい｝ to 可能形｛かのうけい｝. One could say something like the following.
Replace the "-u" with "-eru."
Example: 飲む｛のむ｝ → 飲める
Of course, this is assuming that we're aware of and abide by the linguistic rules of Japanese pronunciation, which means things like that if you have a "-tsu" ending and want to replace "-u" with "-eru", this really means to change "-tsu" to "-teru," not "-tseru."
If you wanted to make a note like this in Japanese, however, it's a little awkward.
It's awkward because this rule really applies to the u/e sounds and not the う・え kana specifically. For instance, if you wanted to apply this rule to the verb 飲む, it might be unclear what it means to replace 「～う」 with something when the kana う is not present.
How, in Japanese, does one denote in a short note form something like the "Replace the -u with -eru." sentence I indicated above?