If you don't know how to do it, don't fool with it.

你要是不会干就别胡来。

《新英汉大辞典》

And you don't know how to do it.

而你不知道怎么去做。

youdao

I don't know how to do it really.

我不知道要多快才行。

youdao

Let us say I want to sell my car but I do not know exactly how much it is worth.

VOA: special.2009.04.12

And yeah, I know you know how to do it, but let's think about what might happen if I wanted to do that.

对，我知道你们知道该怎么做，那么让我们看看如果我,想要完成的话会发生什么。

麻省理工公开课 - 计算机科学及编程导论课程节选

If you know how to do it, you can get it for really cheap.

要是你知道怎么做的话，你就可以买到很便宜的票。

实惠的歌舞剧 - SpeakingMax英语口语达人

The beauty of the trade is that very few people know how to do it right.

FORBES: Sublime Subprime

NPR's Christopher Joyce reports on a new study that says scientists still don't know how to do it.

NPR: Restoration of Gulf Coast Wetlands Poses Challenge

Unfortunately while everyone agrees that bipartisanship is a good thing, nobody seems to know how to do it.

FORBES: Four Steps for Bridging the Partisan Divide