Step 1: Update the source code.
Step 2: Run the current tests.
Step 3: Write new tests.
This seems really slow at first. If I haven't touched a code base in a while and the developers haven't been maintaining the tests*, I often wind up spending an hour or so just getting the tests and the code back to where they were.
A shaky foundation is no place to start testing. So first stabilize, then extend.
* Bad developers! You break the test, you should fix it.