If the sun is actually in the frame (i.e., before it sets), you'll have some issues shooting at your 1/125 sync -- you'll need an aperture of f/16 at ISO 100, and even at that the sun/sky is likely too bright. (Try hiding the sun behind the subject, or letting it peek around and flare a little.)
As for shooting this, the folks above are correct: start with your flash exposure and set the correct aperture for the flash output and subject distance at your lowest ISO. (This can be difficult to do in full daylight as you set up.) That leaves one variable that you can control - your shutter speed. After the sun is down, the sky will still be very bright, but as it gets darker over the next hour or so you can shoot a variety of different looks, choosing slower shutter speeds to keep the background at the level that you want to balance the flash.
I do find that I need to change my aperture and flash power as it gets very dark, opening the aperture (and reducing flash power) to keep a usable shutter speed when shooting people.
Assuming that you're shooting digitally, you'll be able to see a lot of this on the LCD while you're shooting. Makes the learning curve a little less steep
Good luck, and post a photo here if you like....