I'm right handed but my left eye is slightly dominant. There really are three approaches you can take:
1. Use your left, dominant eye.
You will lose some peripheral vision to your left by doing this. Some people train themselves to put their chin on or near their shoulder when aiming.
2. Close or squint your dominant eye so the right eye is used to focus.
If your left eye is dominant because your vision is much worse in your right eye, then this isn't advisable. I slightly squint when attaining the gun sites and then once things are properly in focus and aligned, I can fully open my left eye and maintain peripheral vision without the eye dominance shifting. It's taken a very long time to train my brain and body to do this.
3. Train your right eye to be dominant when aiming.
This takes a LONG time to do and it now works for me most of the time, but I still squint out of habit and to ensure that my left eye doesn't try to take over.
I think the key is to do what is most comfortable and do it enough that you know that's what your body will do in a stressful situation. In a pressured situation, I'd rather fall back on my habit of squinting than rely on my brain to sort out the gun sites appropriately.
While my left eye is slightly dominant, I have never met a person that has better eyesight than I do. I really feel blessed for this - especially because my wife is blind as a bat without glasses or contacts.