put the cart before the horse

这个习语字面上的意思是“把马拉的车放在马的前面”，也就是没有按照正确的次序做事情，把事情搞复杂了。

to do things in the wrong order

本末倒置;前后颠倒

Aren't you putting the cart before the horse by deciding what to wear for the wedding before you've even been invited to it?

还未被邀请参加婚礼，就先决定该穿什么，你这不是本末倒置吗?