First determine your dominate eye.

It's simple, with both eyes open point with you stong hand index finger at a small object in the room. Close your oposite eye, are you looking down your finger at the object? If so you are strong eye dominate and good to go.

If not then you're cross eye domanite and will have to modify things a bit, though not really a problem.

Assuming you are strong eye dominate bring your weapon up to your plain of vision keeping both eyes open. Now while keeping both eyes open allow yourself to focus on the front sight while maintaining it in the center blur of your chosen target and centered in the blur that will be your rear sight.

If you're cross eye dominate you do the same thing only tilt the handgun approx 10-15 degrees toward your dominate eye while tilting your head towards your dominate arm untill the sight comes into focus.

Hope you can understand this as it's much easier to demonstrate this than it is to try and explain it.