In general, the first step for stitching is to find the best rig preset or calibration data for your camera rig model, and use it as a starting point for the stitching. 

MistikaVR comes with a camera presets menu (Scene->Load Preset) including all common camera rigs in the VR market. Alternatively, there are some specialised tools to import calibration data from some camera files (Kandao and InstaPro camera rigs, available trough Scene->Stitch menu). But if your camera rig is a new variant, a new model, or a custom made rig and it is not covered by any of those options then you can try these resources:

If available, use a camera preset for the same camera at different resolution:

Camera presets may  also work at lower resolution than indicated, as long as the aspect ratio is the same and the image is just a downscale version, not the result of sensor masking / central crop. (Some cameras use lower resolutions when increasing the frame rate, but the geometry is identical).  

You can create new camera presets with PTGui or Hugin:

Third party applications like PTGui or Hugin permit  to characterize a camera rig. They provide basic stitch with geometry information and lens distortion calibration. Then you can import this stitch in MistikaVR to create a camera preset for your rig. Here it is a tutorial demonstrating the integration workflow for both products:  

 PTGui:       Hugin:

And a more advanced tutorial here:

There is also a good description is this Mistika VR Quick Guide

Please note that PTGui "masks" are not supported, as they are not compatible with MistikaVR edge points.  PTGUI should only be used to calculate the parameters related with the lens geometry and  the positions and geometry transformations of the cameras.

APG Autopano users can get the calibration data from APG

This is explained in the arrached PDF:

MistikaVR rig presets forum:

New rig presets are included with each new Mistika VR version. Between versions, you may find  anticipated access to new rigs in the Mistika VR Rig Presets.  

Asking SGO and expert users to create a camera preset for you:

You can also ask others to create the preset for you. Please post in the presets forum or open a support ticket for this. You will need to provide sample images, ideally recorded in a place with nothing too close to the camera (a clear in a park, a parking lot, a town square... )