To train vines, such as ivy, on stuffed forms, train the stems on moss by pinning them to the surface. They will send out small roots wherever the shoots touch the moss. Don't pin down the edges. Pin between the leaves, never through them. Train one even layer of growth.
Leaves trapped beneath other leaves soon rot.
Each day the plants will grow. Train and pin new shoots to fill bare patches. When you cover the surface, trim awkward stems with a scissors. Once the topiary is growing, it requires watering, feeding, grooming, insect control, and disease control. Misting will not keep the plants watered.