Well, I can tell you, its not as hard as you think! If you can take a properly exposed landscape in manual mode, then you can balance flash and ambient.
When balancing flash and ambient outdoors (or indoors for that matter), the first thing to remember is that you are essentially making two different exposures at once; one for the ambient portion and one for the flash lit portion. So the first thing you want to do is nail that ambient exposure where you want it. This is highly subjective and can range anywhere from blown out to pitch black, but for starters, lets say you want to balance the exposures as close as possible.
The first thing I do when balancing flash and ambient outdoors, in manual mode, is to set my camera's shutter speed to its fastest flash sync speed. This varies from model to model so you may want to check your camera's manual if you're not sure. The reason I go with my fastest flash sync speed is, its going to give me a broader aperture range to work with, and will allow me to shoot with the biggest aperture I can which in turn will stress the batteries in my flash less which equals more pops of light using less power. Then, I simply dial in my aperture until my background looks how I want.
Note: when shooting outside in sunny conditions, I will always start with my ISO at 200 or lower to get the best possible quality I can, but may adjust it as necessary based on the ambient exposure.
In the photo below, notice I didn't even bother worrying about whether my subject was in focus or even ready for the shot. This frame was simply to get my background (ambient portion) dialed in where I want it.
Sometimes, I may even purposely throw my subject way out of focus to give me a smoother looking ambient background shot which I can check on my histogram. (However, I rarely use my histogram except in tricky lighting situations ...that's what the LCD is for!) chimp, chimp
So in the photo above, my chosen shutter speed, aperture an ISO have my background where I want it and left my subject a little in shadow. Now if I didn't mind a brighter background, I probably could have gotten my subject properly exposed without flash, but the whole point of using flash is to make the subject stand out from the background.
The next step is to bring my flash in and dial the power up or down manually on the flash til I have the exposure I want for my subject without changing any of my exposure settings for my ambient portion of the photo. In this particular photo, I believe the flash power was around 1/16th and it was shot thru a white, translucent, 43" Westcott double fold umbrella (one of my favorite light modifiers).
If you wanted the subject a little darker, you simply dial the power down on your flash, if you want it brighter, just dial the power up ...and that's it!
To be sure, this is not the only way you could have made this photo exposure wise. However its a good starting point to learn how to balance the ambient and flash to get a properly exposed image ...properly being used loosely here.
Both of these photos were taken with the exact same aperture, shutter speed and ISO setting. The only difference is in the second one, flash was added to light my subject. Also, both of these photos are straight out of the camera with only a little cropping done in post.
Hopefully this will clear up some mystery in balancing flash and ambient outdoors, but if you have any questions, please don't hesitate at all to ask any questions in the comments.
For further reading on balancing flash and ambient outdoors, check out these awesome links on the Strobist blog...
Lighting 101:Balancing Flash with Ambient, Pt 2
Lighting 102 - 3.1 Balancing Light: Twilight
Lighting 102 - 3.2 Balance | Flash/Sun Crosslighting
More to come!