Do a search for cake dummies to find various vendors; I personally like the quality of the styro from The Dummy Place in Connecticut. Spritz the outside of the styro very lightly with a fine mist of water just before you put the fondant on so it will stick to the dummy. Cheap styro (with big, loose cells) will show under the fondant, so some people cover that with a very thin base of royal icing. I’d rather get nice styro to begin with!
The easiest way for a newbie at fondant to get a design is to put a decorative ribbon around the side of the cake (either in the middle, just below the top edge or at the bottom). You could also buy a stencil to use, but you will want to practice that to get the clean lines you need and with a dark royal icing, that will take some practice in getting the right consistency of icing and pulling the stencil away.
If you use fondant cut outs, it’s easiest to start with precolored fondant (from either Cal-Java or Satin Ice, both will dry very quickly once rolled, so cover the exposed part with plastic wrap while you work on another part). Use cutters if you can find ones in a pattern you like or create a pattern from different cutters - apply when the black fondant is still pliable - don’t cut all the pieces for all the cakes and then go back to apply them. Cut and apply pieces to sections of the cake as you go.
Give yourself a lot of time; working with fondant, or I should say, working well with fondant - like anything - takes some practice. You can start the dummies months in advance, just store them in a box to keep them out of the light (so they don’t fade) and away from dust and bugs.
Congratulations on your upcoming wedding!