Make sure the Camera Front Surface is Clean when Mounting, also make sure to Remove the Stainless CS-Mount Ring together with The Screw and Clamping Rod when using anything like the Focal Reducer.
Then proceed to Figure out the Correct Shims to put under it.
Tighten The Screws firmly, but dont over torque the Screws, as that can damage and Distort the Camera Body or the Threads inside the Camera.
Proper Torque for Fastening these Screws is about 1,4Nm if you happen to have a small Torque Driver.
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Other than that, there is not much to it really, but if you want, i can shoot a Quick "How-To" Video and post that, so you can follow that in Detail.