Making a mod, how would I start?

A good starting point would be to visit the official resource for Unturned modding documentation. The most relevant article would be the one on the game data properties for Blueprints, but if you're brand new to modding you'll likely want to familiarize yourself with other concepts as well (like creating an item).

I recommend referencing any of the vanilla game data files that have blueprints, which can be found along the ...\Steam\steamapps\common\Unturned\Bundles\Items file path.

The gist of what you're wanting to do is fairly simple. Note that you can't add new crafting recipes without having them be part of an item's game data. The (very summarized) steps would generally look like this:

1. Create a new item. It doesn't need to be obtainable, or even have a mesh – it just needs to be a valid item.
2. Add blueprints to the game data of your item. For each blueprint, you're wanting to specify the vanilla (or modded) item as an input, and then its equivalent modded (or vanilla) version as an output.
3. Make sure your mod doesn't have any errors (visible in the log files, or from the Error Logs available from the main menu).
4. Upload your mod to the Steam Workshop.