Exegesis:

kai hupestrepsan hoi poimenes ‘and the shepherds returned,’ to their flocks; kai ‘and’ indicates that the account of the events immediately following the birth of Christ is resumed after mentioning in v. 19 what Mary’s attitude was.

doxazontes kai ainountes ton theon ‘glorifying and praising God.’

doxazō, always (except 4.15) with God as object, ‘to honour,’ ‘to glorify.’ As compared with aineō ‘to praise’ doxazō is the stronger of the two.

epi pasin hois ēkousan kai eidon ‘for all that they had heard and seen’; epi followed by the dative introduces here that upon which the glorifying and praising of the shepherds is based. The relative pronoun hois ‘that’ has been attracted into the case of the antecedent pasin ‘all.’ The verbs are in the aorist tense but are to be rendered in the pluperfect, cf. Moule 16. ēkousan kai eidon ‘(what) they had heard and seen’ is best understood as one expression summing up the experience of the shepherds in Bethlehem.

kathōs elalēthē pros autous ‘as it had been said to them,’ scil. by the angel, to be connected only with the preceding ‘all that they had heard and seen.’

Translation:

The shepherds returned, glorifying and praising God, or, changing the pattern of subordination, ‘when the shepherds returned (i.e. to their sheep), they were-glorifying, they were-praising God’ (Kituba); or, shifting to co-ordination, ‘the shepherds went back, (and) they glorified and praised God.’ Glorifying and praising. A few other renderings are, ‘to make big the precious quality of’ (Kekchi), ‘to holy-remember’ (Tabasco Chontal). For the second verb see above v. 13. Since in several cases the renderings of the two verbs are basically the same or closely resemble each other, their combination may require some adaptation, such as using one verb with two qualifications, e.g. ‘lift up the brightness and the name,’ ‘say that is great and good,’ or representing the second verb as an indication of high degree, e.g. ‘to speak extremely well of,’ ‘great their praising’ (Tboli).

All they had heard and seen, as it had been told them, or to express the relationship of the two clauses more clearly, “all that they had heard and seen in fulfilment of what they had been told” (An American Translation), “all they had heard and seen; it had been just as the angel had told them” (Good News Translation, similarly New English Bible, Willibrord).

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.