Thanks Jeremy! Yes, it walks through it end to end. ARCHITECTURE.md has an “Adding a weapon” section: duplicate an existing weapon’s .tres, set its name, sprite, stats and per-level changes, then add it to the weapons list in data/config.tres, and it shows up on the level-up cards. There’s an optional step for evolutions too.
If you want a new behavior rather than new numbers, the same section has a short shockwave weapon as a code example. Enemies have a similar walkthrough.