I'm the type of writer/artist who needs to know every detail about his locations. My comic will be a mix of ancient Earth and my own fantasy realm, so a lot of Earth places are real locations (i.e. Africa) and the fantasy realm is based on real locations.
I like to do as much visual and written planning as possible. I'll find blank worldbuilding templates to fill out just to get an idea of what my setting is like -- what are its origins, what's the geography/climate like, where is it positioned near other settings, what kind of creatures and plants live there, are there any civilizations, etc. It gets the most complicated when you factor in civilizations, because then I need to write down how the civilization was created, what's the major form of government, are there any official religions, what's daily life like, etc.
And as I'm writing things out and brainstorming new ideas, I like to doodle some visual ideas too. Here's some concept art I've done so far for the fantasy realm:


I wish I had a neat process for worldbuilding, but honestly I just research, write, and draw until I feel like I know enough about my setting. I highly recommend worldbuilding templates because they're a great place to start brainstorming and work your way up from there. I'd also recommend getting a Pinterest or something similar where you can collect pictures of different locations that will help you create your own.
And I also love to write snippets where my characters explain the settings to other characters -- dunno why, but I find that really helpful and fun. 