You already know how to fix a single flaky test. This lecture is about the situation you will inherit: a suite where forty tests fail at random and everyone has given up. Older Selenium suites are where this problem is most common, so you will probably meet it early.
You cannot debug forty tests one by one. You need a different approach.
Step 1: measure before you fix anything. Selenium gives you nothing here, so build it yourself. Save each run's results to a file and count failures per test over a week. A simple spreadsheet is enough to start.