Hello and welcome on the forum! now, to answer your questions,

souledout escribió

Do you just layer the arms, legs, body and head or is there more that should be done like hands, feet, and neck as well? What if the character is wearing clothes, would you make a layer for a cloak and hat etc?

If a piece is supposed to move, bounce, deform, it will go on its own layer, if it's meant to move together with something else, it's better to put it together with that. If you're going to move the neck, and then the head at a different angle than the neck, you want to have the neck as a separate piece. If you're going to animate fingers, have separate fingers, if you don't care, just have a hand drawing. To sum it up, it's a matter of what you're planning your character to be able to do.

souledout escribió

Do you start with a standing/idle pose and then go from there for walking/running/jumping/weapons animations?

Usually yes, but what makes the most sense is to draw the character in the pose it will have the longest, some characters might be drawn to nap on a couch and grab pop corns, those wouldn't need to walk so you would draw them already sitting. But if they're supposed to go around for most of the game, and occasionally sit, you'd want to draw them in a standing position capable of idling and walking first and have the sitting pose adapt from that instead.

souledout escribió

Is it possible to import reference images in the background of Spine like you can do with Photoshop? Rotoscoping for example

Yes, just import those images as attachments like you'd normally do, you can then deactivate export and select to avoid accidentally clicking on them or exporting them in the final project.

souledout escribió

Also, if you're Unity smart, what's a good canvas size for scenes when painting backgrounds/levels in Photoshop? 1080p x 1080p ?

Not Unity smart, but a good canvas size universally depends on the device you're targeting. If you're targeting mobile you'd need smaller characters than desktop, but if you were to target both desktop and mobile, you'd have to draw the characters bigger first, then scale them down. Actually, it's a good thing to draw them bigger in Photoshop, then export at half, or a third of the size. Scaling down is easy, scaling up is more painful

souledout escribió

How would you make sure the character fits well in that size? Also paint/animate it at 1080p x 1080p and scale it down in Unity?

You can export at the size you want from Spine, so you could work at the size you prefer. Generally, common sense would be that even on an HD screen you'll never see the character bigger than the screen size, but yes you can run tests and then export the character at the perfect size for your project.

By the way, you might find interesting this playlist: Spine Twitch Streams - YouTube

Well, there's the must-see user guide playlist as well! Spine User Guide - YouTube
And new live Twitch streams every Thursday where you can ask me things live: https://www.twitch.tv/esotericsoftware

Well, it's not a template, it's just an A4 I sketched on, activated the rulers by pressing Ctrl+R and started sketching on. I would then export at half the dimension, or even a third if I made something quite small. Don't take this as a golden standard, but I understand that when someone is starting any hint can help so here have my neutral bg with guides document.

Note that you'll eventually end up having to set the rulers origin back in place because Photoshop will forget, that's why the guides are there.