A simple mod that just replaces the cat (or whatever) seasonally can be done in a single patch, so it's often fine to start with an existing one and start making changes. A more complex mod with a ton of assets, configuration options, custom tokens or other things like that will probably just make it harder to understand what is happening, may not be best for your own mod, and will have all this custom work in it that shouldn't be copied without permission, anyway.
Content Patcher has an author guide
here! It's very extensive, so between that, the
validator to check your jsons, and looking at existing packs for examples, you should be able to figure out simple packs pretty fast :) If you've never modded anything ever before, the CP documentation may be a little overwhelming (it can do a LOT of stuff), so breaking/improving existing packs in baby steps may help. You should also
unpack the game files if you haven't already, to get familiar with which files live in which folders and such.
There are also some template packs meant for personal edits and mashups you can search for. If you get stuck or have any questions on specifics, you can always ask here or #making-mods on the SDV Discord server. There are a lot of people that can help get the actual pack organization sorted :)