How paleontologist use fossils of known ages to determine relative ages of other fossils?

Procedure 1 - Ask the famous paleontologist! A person famous in that field would certainly be aware of the stratigraphy of the area.

Procedure 2 - Have a detailed geologic map of the area with latitude and longitude so a GPS could quickly determine your position on the map and thus the age based on the map.

Procedure 3 - Take samples of the rock back to the lab for radiometric dating techniques.