TPMS light has come on with my 2008 Mazda3. Took it to O'Reillys to verify if was the monitor and not a leaking tire; they quoted me at around $400 to replace all four sensors (they're recommendation; only one is actually faulty).

Don't mind doing this myself, but how hard would this be? Looked around but couldn't seem to find same situation or guide. Any helps/links appreciated.