We play modded Terraria a bunch, and we've tried to mod before.
What it does
Adds sans to terraria
How I built it
By working with the tmodloader modding skeleton, vanilla shader library, and creating some c# files in the modloader repositories.
Challenges I ran into
Locating vanilla files to both edit them and figure out the item file formats, deconstructing the game into cs code files, setting up visual studio, and other things. The Terraria code isn't the most straightforward thing to work with, nor is it very intuitive. C# is hard :C.
Accomplishments that I'm proud of
Learning basic python (mostly not used here) and c#, as well as learning the basics of modding Terraria and working with deconstruction of programs.
What I learned
Most of the previous answer