I'm trying to make a mod with a custom biome and the only tutorial I've found is for 1.17

I understand there was a big change in the 1.18 update do you have a more recent guide?

Use the minecraft wiki since it tells you the JSON format and then you can add the custom biome to the overworld using BiomeManager#addAdditionalOverworldBiome or #addBiome during FMLCommonSetupEvent in the deferred work queue (#enqueueWork).