Well, hello there. My name is Ah2190 (at least, that's how I call myself online) and I'm kinda new to the making of custom maps for Mario Kart. I have a tiny bit of experience with modelling tracks, but I currently don't have any experience in converting the model into a proper track. I do, however, have some experience in making circuit maps from my time creating maps for Sonic Robo Blast 2 - but to be more specific, for the Kart mode of the SRB2 Riders mod. And I may see if I could port some of those levels over to Mario Kart. I'm also a big fan of the Digipen 'Experimental Survival Driving' game Nitronic Rush, and also its spiritual successor, Distance - and, while making tracks based on tracks in those games would be hard, especially giving that they are point-to-point rather than circuit tracks and also due to the gravity-defying physics of some of the latter tracks, I may want to make a track somewhat based off of those games.
My current project is a custom-made track in the style of Rainbow Road from the SNES/GBA games, called Nitro Starway.