This was very confusing. Three new words in one sentence and it was one of those tricky word order ones too.
Mousing over the translation was something like "let it yourself go well" which makes no sense, really. I felt like it was one we had to randomly guess, rather than translate.
The correct translation for the question says it is "have a good one" but then for this discussion popup it says at the top it says the translation is "let yourself relax" which seems to be two different meanings to me. My guesswork was "hope it goes well" which was incorrect (which is fair enough, I guess).
I'm an Australian, we do say "have a good one" meaning "have a great day" or "have a good time" but it isn't the same as saying "let yourself relax."
I guess I am just really confused how to get from "let it yourself go well" to "have a good one" from "Lass es dir gutgehen!"
I think you mean 'make yourself at home' ...
I suppose this sentence is an idiom with no direct English translation. I agree that 'Let yourself relax' is an odd saying, I would only ever use it if somebody was stressed I suppose?? Maybe a better translation would be just 'Relax' or 'Chill out' or something like that.