Start with your support hand on your chest. Yes, really (remember - never point the muzzle at anything you aren't willing to kill, maim or destroy - including your support hand!)
As you pick up the pistol, the trigger finger should be high and straight along the frame. No portion of your trigger finger should be seen inside the trigger guard. Your wrist should be cocked slightly forward as if you were shaking hands with someone.
The web of your hand, between the thumb and your trigger finger, should be as high on the backstrap as possible
Wrap your lower three fingers tightly against the frame and as high under the trigger guard as possible. Your pinky should be in contact with the frame. You shouldn't feel strained to hold the pistol with one hand. Your trigger finger should comfortably reach the center of the trigger.
Again, cock your support wrist forward as though you were shaking hands with someone. Bring the palm of your support hand in to fill the gap between your lower three fingers and the palm of your trigger hand. Lock the fingers of your trigger hand in place with your support hand fingers.
Keep your trigger thumb elevated or pointed at the target. Ensure that your trigger thumb is not being pinned by your support palm.
Close your eyes and take a few breaths to settle.
Lean forward so that your nose is over your toes and you are on the balls of your feet. Extend your arms until your elbows are not quite locked out.
Open your eyes - if your sights are in alignment and on target; you have a good fit.
If the front sight is high, you need a smaller backstrap. If the front sight is low, you need a larger backstrap. Some pistols have inserts that you can use to adjust the backstrap.
It is possible to train through any poorly fit pistol. Sometimes the advantages of a smaller (more concealable) or larger (more capacity, less recoil, longer sight radius etc.) pistol outweigh the imperfect fit; however, you will be more accurate with a properly fit pistol.