To Unity and Beyond
Open the Avatar Unity Project.
Import the fbx file with your waifu, and create an empty game object with her name, but don't drag her in yet.
Next step will be to click on the Avatar asset, and set its rig as humanoid, and then configure it. If you've worked with VRChat avatars before this should sound familiar, so I'll skip the configuration, which basically is just match the body parts and enforce T-Pose. After doing that you can drag your Avatar into the GameObject that you created before.
Create 4 Empty GameObjects in the root object, and name them Head, Body, LeftHand, and RightHand.
You want to position Head where you want your camera to be, similar to VRChat's white dot.
For both LeftHand and RightHand, you want to position them on your Avatar's hands like if they were your controllers. The positive Y axis points up from the controller, so you want that to point towards your wrists.
Make sure you have your GameObjects where you want them, after the next step their position is final.
We're ready to start the fun! Navigate the hierarchy of the Armature until you reach the Head, select it and right-click and Create Empty, then drag it into the Head we created and name it HeadTarget. That process will make sure that it's in the correct place.
Do the same with your wrists, placing them into the LeftHand and RightHand, and name them LeftHandTarget and RightHandTarget respectively. It should look something like this.
If you need to move Head, LeftHand, or RightHand AFTER you've already placed the targets, you'll need to delete the targets and add them again after you're done moving them.
Select your Avatar, the one that is blue in my previous image, and add some components to it: First you'll need VR IK and IK Manager, or you can use IK Manager Advanced if you want to change the IK settings. Add the 3 targets you just created to the IK Manager of your choice.
You should have a fully functional avatar for Beat Saber, but don't hit that export button yet, we still need to give it the finishing touches. Once you're done testing out the Avatar in Play mode and IK is working, you'll have to set all your materials to use one of the included Beat Saber shaders, if you've atlased your materials, great, you get to skip the painstakingly arduous job of retexturing each one of them, because for some reason those shaders don't take the same texture parameter as most other shaders.
You might have noticed that your avatar crosses its legs like it needs to pee when it crouches down. You might have played around with the IK options and noticed that changing the Swivel Offset on the legs fixes this in unity, but the changes don't appear in the game. Now you can fix that by using IK Manager Advanced. Adding the swivel offsets to the IK Manager Advanced component will make those settings work in the game. If you haven't played around with those before, they change the angle at which the knees bend. A positive value moves the knees to the right, and a negative value moves them to the left, from the avatar's perspective.
You can now go ahead and do cosmetic stuff to your avatar, add dynamic bones to your liking, but use the included scripts. You can also curl your waifu's fingers so she's actually holding the sabers, I used Shape Keys for that. Avatars are now visible in first person view. You can toggle it using the Home key on your keyboard. If you want to be able to see your avatar in first person, but don't want to see the inside of your head, you can use the First Person Exclusion component to exclude your head's mesh, note that it will have to be a separate mesh for this to work. If you have double vision of your avatar, like if you were cross eyed, you need to change a setting. Go to Edit -> Project Settings -> Player. On the window that appears click on XR Settings, Make sure that Virtual Reality Supported is turned on, and that Stereo Rendering Method is set to Single Pass.