CATHEDRAL ASSETS OPTIMIZER
Meshes made for Classic Skyrim often need to have some parts removed or be otherwise converted for SSE to prevent incompatibilities, visual bugs and crashes. Cathedral Assets Optimizer does that as well as unpack / repack BSAs and compress textures to take up less space. You will need to run it on the mod folder of pretty much all Classic Skyrim mods you install in the guide.
This tool was formerly known as SSE Asset Optimizer and Bethesda Asset Optimizer.