This is a bracket for installing two 2.5" hard drives or solid state drives in to a single 3.5" hard drive bay. Optionally, it can be used to mount one 2.5" drive on top and a 50x50x10mm cooling fan below it.
If you are only mounting one drive without a fan and you'd like a cheaper/faster to print bracket, I have put one online which can be found at https://www.myminifactory.com/object/3d-print-hard-drive-bracket-short-111938
I also have my longer version for scenerios where you need all three mounting holes which can be found at https://www.thingiverse.com/thing:4151949
To attach the drives to this bracket, the top hard drive will screw in from the side, and the bottom hard drive or fan will screw in from the bottom.
The holes on the side of this bracket for attaching it to the case are sized to allow you to self tap a #6-32 UNC screw. (The standard ones that are used with 3.5" hard drives, not the M3 screw which has a finer thread and is generally used for optical drives and 2.5" drives - although if you find the hole too small for the 6-32, you could try an M3.)
Please tip if you can afford it and this is useful to you :)