Sweet, if I was you I'd start simple.
This is my advice:-[list type=decimal]
[*]Send the ST-E2 back, they are very limited in functionality and reliability, especially how you are trying to use one, outside in full sun, inside modifiers and off axis, just about the worst situation for the ST-E2.
[*]Buy a set of RF-603's, they are dirt cheap, very reliable, etc. They are just dumb triggers but all my previous images were shot with them.
[*]Buy this book,
http://www.amazon.com/Speedliters-Handbook-Learning-Craft-Speedlites/dp/032171105X
[*]Don't buy anything else until you have read the book, seriously it should come free with every Canon Speedlite.
[/list]
Now when you go to your shoot set your camera like this. Manual mode for both camera AND flash. Forget ETTL for this initially, I promise manual is the way to get your head around it, trust me.
Set camera to M, 100iso, 1/180 shutter, f8. Take a picture with the flash off. How bright is the picture? If you want the ambient darker then close your aperture, if you want it lighter lengthen your shutter speed, you do not need your model in the shot yet, we are just working out ambient exposure. Get your ambient exposure dialed in, if it is setting sun watch the light it lowers fast so you need to keep lengthening your shutter speed.
Now get your model in position, take a shot, is the ambient still right? If so turn the flash on at 1/2 power, use as little modification as possible, no double diffusers etc, and put the flash as close to the model as possible, even 6 inches can make a huge difference. Take the picture, if your model is too dark raise the flash power, if they are too light lower the flash power.
That is the basics, once you nail that then start playing with the two exposure triangles I mentioned earlier in the thread. For instance if both ambient and subject are too dark raise iso or open aperture.
But get that book!