I had a lot of gumpaste callas to make the other day, so here's the process:
Roll out gumpaste to a 4 or 5 on a pasta machine and cut out the callas using a calla cutter. You can also use the one that comes with the former you'll use later, but I like the shape of this one better.
Thin out the edge using a ball tool. I like the ones that are made for polymer clay because they're more solid than the ones they sell for cake decorating.
Wrap the flower around the former, with the pointy end down. Overlap the two edges and pinch the base so that there's a thicker area of gumpaste to hold it on the cone. You might have to use a little gum glue to stick the sides together but you might not, depending on how soft the gumpaste it.
Curl the edges of the flower out and the tip up from the cone. Look at a picture of a real bouquet of calla lilies to see how much you want to curl them.
Let them dry on the formers for a day or two until you can EASILY slide them off of the cones. Remove them by twisting them gently and pulling at the same time. If they don't want to move, don't force them or they might crack. Let them dry a little longer.
For the centers, make little sausages of gumpaste and let them dry while the flowers are drying.
To attach the centers to the flowers, use some royal icing or watered-down gumpaste glue as to attach them inside the flower cone.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA and online cake supplies at www.acaketoremember.biz and www.acaketoremember.etsy.com