FatherToast

You need to update your rendering code to use the new Icon objects, since integer icon indexes are no longer used. Look in RenderBlocks to see how vanilla does it.

We have no idea what or where these things are. What class are they in and what is the method being called from? What are the method's arguments? Return type? The fields' types? If you know those, all you do is search for another method or field in the location with the same return type and arguments, there can't be a ton. Same with the field, but you're more likely to find more than one field with that field's type.

I'd suggest you make your own render file similar to RenderArrow. If you don't mind how silly your dagger entity would look with the standard throwable renderer, just use RenderSnowball.

Data watchers are used when you need to send server information to the client or client info to the server. In most cases, this is going to be a variable determined server-side that you want to use for rendering. In your example with the wolf, the health is only calculated on the server side, but the wolf's tail needs to render at an angle based on its health. In some cases, the variable will be calculated the same on both the client and server sides or the variable is only needed on the side it is calculated by, so data watcher is not needed.

The mobs in my mods act very strangely; they walk around sideways and when they are attacking me, they move very slowly and keep twitching between looking at me and looking in an arbitrary direction. There are a few other odd things that happen, like I can't see lightning spawned by my entities. I've never done a server mod before, can anyone tell me what I need to do to make my entities work properly? I'm registering my entities and their spawns with both ModLoader and EntityRegistry, but I don't know what else I need to do for both the server and client or how to do it.