Drawing a crystal takes 3 steps:

1. Select a pointgroup
A pointgroup is a set of symmetry elements; it determines the symmetry, and thus the appearance, of the crystal. Just pick one from the dropdown list.
Each pointgroup belongs to one of the seven crystal systems. If you're not sure what to do, start with 'm3m'.

2. Enter unit cell properties (celldata)
The unit cell is the basic building block of the crystal structure. It has the shape of a parallelepiped.
The unit cell edge lengths a, b, c and interedge angles α, β and γ are prefilled with the values of a cubic unit cell.
If you change a value, click the 'Draw crystal' button to redraw te crystal.
You can also select the cellparameters for a specific mineral.

3. Enter crystal forms
A crystal form is a set of crystal faces that are related by symmetry. Click the 'Add form' button to add a crystal form.
Forms are designated by Miller indices, three integer values h, k and l.
Forms can be deleted by clicking the 'Delete' button next to it.
If your crystal has more than one form, you can change the relative size of each form by changing its distance to the center of the crystals. Enter a new value in the 'Distance' field, or use the little arrows next to it to change the size on the fly.
After adding or deleting a form, click the 'Draw crystal' button to redraw the crystal.
Note that a crystal shape must enclose a volume; if the combination of crystal forms you entered isn't closed, it probably will not be drawn.