i've only ever run premade modules or googled premade battle maps and such, where should i start if i want to make a dungeon from scratch

One Canuck built the #ttrpg tag and the #mecha tag. And that was me.
Cohost Cultural Institution: @Making-up-Mech-Pilots
Priv: @Scampriv
Also! Ask for help all the time! Google Stuff! Watch a
Mapcrow video and turn your nose up!