Topiaries are often formed on wire frames. Plants are attached to the frame with wires. Plants are also carefully pruned. Some topiaries are only pruned, like yew trees.
Pick a shape and start off with a 45 degrees shapes in that angle. Then cut an inch with a pair of shears. Plants should be cut one third of the way for pruning.
Most people use wire to form a frame to grow the topiary on. You can make it yourself, or if you are going for a small sized one, many craft stores carry them.