On the flop I mostly use donk betting in multiway pots, where the preflop raiser is unlikely to C-bet with air, and where either the stack to pot ratio or my hand strenght is such, that there is no real benefit to a check-raise. On the turn and river I use donk bets, when the texture change. A simple and typical example would be, when the river complete a frontdoor flushdraw. In that situation people tend to check back even very strong non-flush hands, so if I want to get paid, a donk bet is often my best option. Of course this is a very obvious play, but if you notice, that people fold a lot to a river donk on a flushy board, then just start mixing in some bluffs.