Melt a little marmalade in the microwave or on the stove, until it runny. Brush this over the cake, it will stick the marzipan to the cake. I find the easiest way to get the layer of marzipan draped over the cake safely is to first drape it over a rolling pin, and then roll it over the surface of the cake. Dust it with some icing sugar or cornflour first to stop it sticking.
Sugar flowers are beautiful things. With care and patience they can be delicate fascimiles of real blooms, created using an edible medium which will last for months if kept dry and cool. As well as cakes they can make good display pieces, a sort of floral sculpture. The flowers are made from flowerpaste (gumpaste) , a very elastic sugarpaste that can be rolled and mainpulated into realistic petals that dry hard and brittle.
You should use a recipe specifically designed to be a wedding cake as it will ensure that it is sturdy enough and that it makes the right amount of batter and icing. For a three-tier cake, you need to make three cakes of 12", 9" and 6", as well as a massive amount of icing. Most three-tier cake recipes have a step-by-step schedule for when you will need to bake and assemble each part.