Here is a nice article on how to write good referee reports on journal articles. The article is by Henry Roediger, a distinguished psychologist at Washington University in St. Louis (thanks to his colleague Jeff Zacks for the pointer), so it focuses on psychology. But mutatis mutandis virtually all of his advice can be tranferred to philosophy refereeing.
Two minor additional comments:
1. I think in philosophy (unlike psychology) it’s very important to always begin a referee report with a summary of the paper, to make it clear what the referee thinks (rightly or wrongly) is the main thesis and argument of the paper.
2. I like the advice to sign referee reports (so the author knows you wrote it) but you might want to wait until after you get tenure before you make this a habit.