do (one's) bit

To contribute work to that of others in a group. If you don't do your bit with the slides, our presentation will never be finished on time. It's a group project, so everyone needs to do their bit to make it successful. I did my bit—I typed up the hand-out, just like you asked!

do your bit

make a useful contribution to an effort or cause. informal

The exhortation to do your bit was much used during World War 1, but the expression was current in the late 19th century.

do your ˈbit

(informal) do your share of a task, help a cause, etc: Everyone is expected to do their bit to make the business successful. ♢ In reusing this old paper I’m doing my bit for conservation of the rainforests.