I spent weeks researching romano-celtic Britain for Errant. Even had a trip to the Roman bathhouse in Bath (Dedicated to Sulis Minerva, the favourite deity of a certain blue haired best girl) which was really great. Also I read a lot of stuff about Arthurian legends and folktales about demons and fey too, obviously, and latin swearwords (though Podex was the only one I ended up using). Plus research into old British place names and how pronunciation of place names shifts over time.
On the style side, I collected reference about the colours used in old American comics and used them as a starting point, doing several style tests before starting.
Oh also, here's something I did that everyone should do. I checked online if there were any other webcomics already with the name I wanted to use to make sure mine was unique.
Honestly I probably did more than I needed to hahaha! 