The base of any fruit cake recipe is the fruit mix. The cake batter is essentially just there to hold all of the dried fruit together. Although I have provided the recipe I used, as long as you end up with roughly the same total weight at the end you can alter the proportions to your taste. In this recipe I was short on mixed peel and currants, but bulked up with dried apricots scrounged from my baking cupboard. It's a very flexible type of cake.
Depending on what kind of filling you're using, it can usually be made well in advance. But some frostings need to be made closer to when the cake will be served. Between the recipe and your practice cakes, you should have a sense of how early the filling and frosting can be made and how long they'll hold up. Once again, overestimate how much you need. Plan to bring an extra pastry bag of frosting to the wedding site to patch up any little cracks that happen while transporting the cake.
For me, the cake is the easy part. But I don't have the patience or skill to make thousands of fondant flowers or smooth buttercream perfectly. That's why I always choose foolproof, classic decorations like satin ribbon (be sure to cover the back with clear plastic tape so the fabric doesn't become greasy, and secure with a pearl-headed pin) and fresh flowers. Covering frosting with shredded coconut or white chocolate curls is another easy way to hide imperfections. I've also seen a gorgeous wedding cake covered in rainbow sprinkles. This frosting that looks like a cloud is simple but looks dreamy. Another rustic style I really like that favors the lazy froster is having a "naked" cake, with filling and frosting on the top, but little to no frosting on the sides. Choose decoration you feel comfortable doing and remember that simple is usually better.
Once the marzipan is draped over the cake, gently pull down the sides to cover any gaps. Smooth the flat of your hand down the sides, pressing the marzipan to the cake and amoothing out any creases or overlaps, Rub the flat of your hand or a cake smoother over the top firmly to flatten and smooth the surface. Smooth out the sides in the same way. Trim off any excess marzipan around the bottom of the cake with a sharp knife and peel it off the board. This can be kept in an airtight bag or container for use next time.