In all my years of ordering pizza, I've never had to do so in sign language, but this page from William "Dr. Bill" Vicars should give you a variety of ways to do so.

Even though you may never have to order in sign language, this info could come in handy if you work in the pizza industry and need to recognize the various ways that the deaf community may sign for it.

An aside: The town I grew up in was also home to the Kansas State School for the Deaf. When I worked at the movie theater concession stand as my after-school and summer job, we had a number of deaf patrons come to see films. I ended up taking a class on American Sign Language, which helped immensely when they'd come in and order. After a while, word must have gotten around in the community that the nerdy dude with curly hair knew the signs for various snacks, because whenever I was on duty they'd make their way to my line. Popcorn and butter were my favorite things to sign.