Rat walking in bones here.
If it is your first time, I honestly recommend outsourcing other people to make the things for you. And then make something small on your own.
For my chest piece I just wear a cow pelvis. Nothing fancy. Just had a friend who had one as a lawn decoration... For some reason... So I put it to use.
For my abdomen I used smoked lamb bones that I picked up from where I work at a pet supply store. I had my co workers give them to their dogs and chew on them a bit to clean off most of it and give them that beat up and chewed look.
In between those and as random accents I grabbed some 1" - 2" cut beef bones, and had dogs clean them out the same way.
After collecting the bones I did what has already been recommend here, soak, scrub, repeat.
If you end up using any sort of bird bone DO NOT COOK IT IF YOU PLAN ON DOING THE FOLLOWING*.
Next I pieced out where I wanted each piece to go and drilled holes in them. Be very careful with this, as bone is SUPER hard and sometimes your drill will try to turn the bone with it instead of digging in deeper, so make sure it's clamped down good and you're using enough pressure. The smaller drill bit you can get away with the better, just to keep the bone more intact, however if you're going for a more savage look, bigger holes, cracks, and thick rope/leather might add to the astetic.
*Bird bones are hollow, cooking the bones makes them frail and much more likely to splinter. Splintered bones are not only difficult to use in crafts, but a splinter of a chicken bone is not something you want to get launched into your eye. If you are considering using chicken or turkey bones I recommend trying to just tie leather or rope around them.
Once all the holes were in place I strung everything together. For my purposes it was for a golem so I actually used a very fine ropelight to tie everything together and plugged it into a battery pack that I had tucked into my shirt that I wore under it. By using something firm like that it kept them in place while also giving me the dangle and bone clanking I was going for. Also I never actually drilled holes in my chest piece, I just tied the ropes through it so I can use the cow pelvis for other projects later if needed, however it would be much easier to have just drilled holes.
For the mask I just got something off of Etsy, as I didn't feel like getting on a government watch list for googling where I could find an actual human skull.
I also had horns, which I also just got from my work, elk or deer antlers, naturally shed are small enough to use as MWE horns. They are a little heavy to be worn without a lot of support to keep them up right but can be awesome if you can figure it out. They are also damn near impossible to hollow out, so I don't recommend that at all.
Like all projects make it your own, do one piece at a time and experiment. Don't fear failure. And most of all, be safe if you are using tools and materials you haven't ever worked with before.