For a wedding, your monogram should not be used until after the ceremony. So on printed materials like the invitation it's important that you don't use the full monogram, but for printed materials at the wedding you can. Using just your first initials in the invitation is acceptable.
There are many ways to go about the process of creating a monogram. You can draw it yourself, use a computer program or a monogram designer. For her wedding the bride and I worked with Caroline at Monogram Inc. in New York.
The bride and groom will use their full monogram on the printed wedding materials as well as items in their home such as beautiful monogrammed linens, like below: