Here's how I learned how to make this:
First I made a character sheet with no automated functionality just by laying out editable text fields and sliders, and adding locked fields as labels.
The first thing I coded was the clear button for the portrait. A canvas can be drawn on by default, but there's no way to erase, so I looked at the documentation for canvases and found the "clear" function, then made a button that calls it for the portrait canvas.
Then, I went through elements of the character sheet one by one, deciding if I wanted them to have any fancy functionality or if a raw text field would be sufficient. If I decided that fanciness was warranted, I looked through the Decker documentation until I figured out how to do what it needed. In a couple cases I had to ask for help on the decker forums.
I was helped by already knowing how to code, although I found Lil (the programming language Decker uses) to be very different from all the languages I knew in several frustrating ways. I first learned to code extremely slowly over several years of exposure throughout my early childhood, using scratch.mit.edu regularly as my primary source of entertainment and social life. I started out copying big pieces code from other people's projects whenever I had to do anything complex, as was encouraged through Scratch's built-in remix culture. Later on I started to grasp more abstract concepts of programming.
If you'd like to copy the code of anything in this deck for your own, you have my permission.