David, for outdoor fill-in shots, there will be a setting on the camera called slow-synch flash, or similar. This is the one you should be using for these outdoor shots. Leave the flash in E-TTL (or whatever mode Canon call their pre-flash TTL mode).
For your indoor stuff, on-camera flash will make everyone look like cardboard cut-outs. You can try either slow synch indoor, depending on how much light there is, or you could switch to fully manual (camera nad flashgun) and do some quick test shots to get the balance between ambient light and the flash lifghting. This will be ok if the interior lighting levels do not change, but as it appears you're not too comfortable with flash, so it may be worth doing some experiments indoors with the family at home before hand to learn how to balance the two light sources.
A longer shutter speed will give more balance to the ambient light levels, but may result in blur due to subject movement, a higher flash power setting will give more balance to the flash lighting, but may result in cardboard cutout effect.
Practice at home using the different modes, and varying the flash compensation setting. It costs nothing to try, and you'll learn a lot in the process. Get comfortable with what you're doing before the big day, even if you are only doing this as a guest.
Duncan
Hope this helps.
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