The following nGram suggests that dos and donts, do's and don'ts, and do's and don't's are all used, but it appears that do's and don'ts takes the cake. A quick search suggested that capitalizing all but the s is also fairly popular:
I want to write "dos and don'ts", or an equivalent expression, in a university essay. I am assuming that it is not very academic to write "dos and don'ts", so does anyone have a better way of putti...
phrase requests - Formal alternative for "dos and don'ts" - English ...
The other quotations use these spellings: Fair do's, fair do's, fair dos, Fair do, fair do's. The first of these is from 1941: 1941 L. A. G. Strong Bay 168 Come on, Doctor. Fair do's. The spellings are usually a variation of the noun do (under which the phrase is listed in the OED), because a do is something done, a dealing or treatment.