Nice set of equipment you've got there!
Before I start, I must stress that I'm NOT a professional photographer, just a hobbyist who reads a lot

. I know about the basics, but because I have no experience I might turn out to be so full of ****
First of all, you'll need to scout out the location where you'll be shooting with the bride and bridegroom. It's your first time - don't stuff it up. You might want to get a rough-and-ready notebook, note down potential location. You might be expected to direct where to photograph, not just merely photographing. You might also be expected to bring your clients to nearby area where there is specular scenary - scout out and take note. The main purpose of scouting out is nto only to check potential spot, but also have a good knowledge of the lighting at the area where you'll be shooting.
If it's going to be in a church... well, someone professional in the forum will be able to help you out better than I can :lmao:
Have a keen eye on the weather. Know what to do when needed.
You'll definitely need a diffuser of some sort, which can be attached on the lens. Others might be able to help me out on this one, because I don't have it. Diffuser basically soften light passing through the flash - useful for night-time shots if required, or any situation where using the flash alone will make the subject look awkward (especially when the background is dark-ish) . Otherwise you might end up casting harsh lights on your clients. If you're shooting in-doors, take note of where you can bounce your flash... note that the colour of the surface where you can bounce your flash will the colour on your subject, but that can be fix using Photoshop.
Do use flash even during day time - you might get some unexpected good results. And extra batteries for your flash should be a must
I'm pretty sure the max high-speed sync for the 350D is 1/200 sec - might turn out to be a handicap.
Hopefully the other folks here could help you out better than I can. And good luck with your shoot!