Mounting is the process of fixing the specimen inside the CT scanner. The important points to remember are to 1) use materials that are less dense than the specimen, 2) make sure the specimen is secure and will not move during scanning, and 3) that the area of interest is at the center of rotation.
The positioning of the specimen is important for getting a high resolution scan. The closer a specimen is to the X-ray source, the higher the resolution or smaller the voxel value will be.
Setting parameters appropriate for your specimen will result in a scan with good penetration and contrast. Parameters mainly include voltage, current, and timing.
Calibration normalizes the detector panel for the parameters you have set. In other words, calibration allows the panel to compensate for bright or dark spots so that the reading is even across the panel when your sample is out of view.