Special Thanks: Raddoob
DISCLAIMER: This video assumes you already have a custom blender rig and know how to use blender.
True rig = The rig that you select when exporting from Blender into Roblox Studio.
Custom rig = The custom rig that you typically animate on with any constraints like Inverse Kinematics/IK and Child Of constraints. The true rig's bones copy the transforms of the custom rig.
NOTE: Rigging an object to the HumanoidRootPart allows you to freely animate the object without having to worry about the objects moving with any other body parts
The plugin to import/export from Roblox Studio & Blender is Cautioned's Blender Animation Plugin
I made this tutorial for a friend who needed assistance converting Roblox Studio rigs into Blender rigs so they could animate it in blender and have it work in Roblox Studio. There might be faster methods, but this is the one I use. Make sure everything such as the plugin and Blender/Roblox Studio are up to date.
SONG: The Stardust Man Appears