Hello. I have an item that I would like to produce particles when an entity is left-clicked with it. I'm not quite sure how to do this.

I have a new class for the item, and I just don't know what to do for the particles.

So, what you are searching for is PlayerInteractEvent.EntityInteract, which fires when the player right clicks an entity...just handle the event and check if the player is holding your custom item. Then use world#AddParticle to spawn particles. You can create your custom particles or use vanilla particles, which you can find inside ParticleTypes class