The teacher underlines each mistake and writes a symbol or an abbreviation next to it, showing the kind of mistake.
Example
‘We made our homework' - WW (wrong word).
In the classroom
Typical codes include WO (word order); WW (wrong word); T (tense); and P (punctuation). Teachers use correction codes to develop the learners' ability to correct and edit their own work, as well as making learning from errors more memorable. Correction codes need to be adjusted to level and also to the degree of learner self-correction expected. For example, for higher level learners who have been editing their own work for some time the teacher can indicate that there is a mistake in a line, and not say where or what kind.
Further links:
https://www.teachingenglish.org.uk/article/error-correction
https://www.teachingenglish.org.uk/article/error-correction-1
https://www.teachingenglish.org.uk/article/error-correction-2