When the hard part of what you are trying to do is in the many small details, you can say "the devil is in the details.” 中国有“失之毫厘，差之千里”一说，英文中则有这么句谚语“The devil is in the details”，用“魔鬼在细节中”来表示细节的重要，有时候决定着成败。所以当你花了很多时间和精力在细节上时，就可以说“The devil is in the details.”

Examples：

I thought I would be able to write that article in two hours, but it ended up taking me five.The devil is in the details. 我本来想我可以用两个小时写这篇文章，但是最后用了我5小时。细节很要命哪！

I can sketch a basic outline of the plan for you and it may look very simple, but the devil is in the details. 我可以给你一个计划的大致轮廓，虽然它看去如此简单，但是细节决定成败，（花了不少心思）。