how to identify expensive steps in an explain analyze output

I've got some long running queries, and want to tune them.
Using simple logic, I can understand what expensive steps in the query plan ought to be (seq scan and index scans using much rows), but I want to quantify; use a somewhat more scientific approach.

The manual states: "Actually two numbers are shown: the start-up time before the first row can be returned, and the total time to return all the rows.". Does this mean that the difference between the first and second is the cost or the time the step in the explain has taken?

TIA

frits