Homemade garum is a fun kitchen experiment, if you have the equipment and patience. Heat + protease + protein substrate is really all you need.

On that note, the easiest way to get your hands on some protease is to buy digestive enzymes sold as food supplements (most often they're made out of dried pork pancreas).

You also don't need much equipment: scales and an immersion circulator should do the trick.

it hasn't updated in a while but i quite like this blog: https://www.culinarycrush.biz/all/will-it-garum