The same goes for investigating. Want to know the proper way to investigate a crime scene, how to preserve evidence, how to interview someone, how to interrogate a suspect, how to do surveillances etc., there’s some expert out there who wrote a book about it.
Now with the internet, you can find these books on-line. You can also contact a legal book store near you for assistance. You can also locate the expert authors themselves on-line and contact them directly. Most of them would be only too willing to share their knowledge and assist you on a case. The key thing is to be pro active. So if you want to learn your trade and get good at your craft, read a book about it.