What is an important idea that geologists use to put sedimentary rocks in order from older to younger?

In a normal pile of sedimentary layers, the bottom layer would be oldest whereas the top would be the youngest.

Sedimentary rocks are formed when layers of sediment from the Earth are being deposited. That means that the bottom most layer would probably be also the first sediments that fell in the area, thus making the top layer the newest sediments.