How to make a Box Shape
First you need a box plant preferably larger than the shape you plan to make. If you pick its shape carefully you can make an instant shaped box.
The easiest shape to make from a box plant is a cone. If the plant is in a pot you can use the pot as a guide for the circle at the bottom. Put a few canes round the plant meeting at the middle of the top of the plant and touching the edge of the pot at the bottom to make a wigwam shape. This gives a guide for where to cut.
Then take a deep breath, get the secateurs or hedge trimmer out and start cutting. If you follow the lines of each cane upwards this gives the outline of the cone. Then either add a few more canes to cut to or just go for it. You should now be the proud owner of a box cone. It will probably look a bit bare and sad, but as long as you have the basic shape you want it will be fine. In a few weeks a few more leaves will grow and look really good.
Even if it looks terrible all is not lost. Either put it out of site and leave it to grow back and try again next year. Or bluff it out and make up a name for the shape that you really wanted to make in the first place, eg. a welsh hat or an oast house.
The other easy shape to make is a ball. First you cut the profile you want in a piece of cardboard then get trimming. The big plus of a ball is that this is the natural shape of a box plant.
Many of the balls and cones you see for sale are cut by a special machine that rotates a shaped hedge cutter round the plant. That is why they are so uniform. I feel that it is a mistake to try to imitate this hard shape, but go for a slightly more organic look.
The big plus of shaping your own is that you know how to trim it in future and it costs very little.