Exegesis:

prosechete heautois ‘be on your guard,’ cf. on 12.1, best understood as an introduction to the exhortation that follows (cf. Plummer, Lagrange).

ean hamartē ho adelphos sou ‘if your brother sins (against you),’ with eis se understood from hamartēsē eis se in v. 4, hence “if your brother wrongs you” (An American Translation). For adelphos cf. on 6.41.

eptimēson autō ‘rebuke him,’ ‘take him to task.’

kai ean metanoēse aphes autō ‘and if he repents, forgive him,’ syntactically parallel to ean hamartē etc. but semantically the point of the exhortation: if your brother wrongs you and if, upon being rebuked by you, he repents, you shall forgive him. For metanoeō cf. on 10.13.

Translation:

Take heed to yourselves, or, “keep watch on yourselves” (New English Bible), ‘take care what you do.’

For sins (i.e. against you) see v. 4; for to rebuke see on 4.35.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.