Have you been to a show and stood ringside? For me, one of the most confusing things was following the flow of the different classes and knowing when to go in the ring. If you don't have anyone with you to help, the steward may be able to keep an eye on you and make sure you know when to go in (they have done this for me before without asking - I must look like a newbie
If the dog is already trained that will help immensely. Just practice practice practice, and make sure it's in different environments - outside, inside, a parking lot, etc. Is the show indoors or out? I find outdoor shows to be much easier because there is so much room to move and gait, so if it's indoors, make sure you practice in a small area to be prepared for that. If it's outdoors, don't worry about it so much and just find the speed you like for your specific dog.
Make sure you've thought through the type of bait you want and how you will carry it. My dog is young and easily bored, so I like to bring a decent amount of bait in the ring if it's a larger class or BOB. I realized in my first show that none of my skirts have pockets, so it's trickier. This is something I didn't think through until the morning of the show, since I had always practiced wearing pants (with pockets) or a treat pouch.
When you arrive at the show, go to your ring and watch what the judge is having other people do. Repeat it to yourself (down and back to the number sign, or whatever) because it's possible to watch 15 dogs go and then still forget what to do when it's your turn. Ask me how I know
I think the best thing is to practice with someone who knows what they're doing, so working with the breeder is going to be really helpful. Good luck and have fun! It seems really overwhelming at first (at least it did to me) but after a few shows it was much easier and I knew what I had to do.
ETA: One more thing. Practice specifically what you will do at the end of the down and back, once you arrive at the judge. I feel like I always put so much thought into the down and back itself, that I arrive and am awkward. Make sure you are stopping far enough away from the judge, and not just running at them and stopping at the last second before you run them over. I think most breeds free stack, though in spaniels that doesn't seem to be the norm and the judges will get the dog's attention to view their expression (which takes some pressure off of me to do anything).