Explain how scientist use fossils to support the theory of evolution?

It's a fine bit of circular reasoning that requires that fossils found fit into the pre-decided place on the evolutionary chart. They then point to the fossil record as proof of the theory. There are many gaps and instances of "parallel evolution". For example, the focusing eye had to develop at least four separate times, for the evolutionary theory.

Answer:

Fossils are like the clues the police use to solve a crime.

Each one tells a story. Like clues to a crime, where the police don't have to find a print of every footstep taken by a subject or confirm every thing that he did, the clues paint a logical outline of all the events around the crime.

Like clues in a crime the police do not initially identify a person they want to pin the crime on and then seek to tie him to the event - they find a number of clues which identify potential subjects of interest. They then attempt to prove the relation of these suspects to the crime until the trail of evidence is strong and dependable. Usually multiple facts on the same evidence are considered - for a crime it might be fingerprints and video tape and eye witnesses, for fossils it could be carbon 14 and dendronchronology and geologic strata.