You order them from the gunsmith in the tower but to order them you have to achieve at least rank 1 with him. To rank up with the gunsmith requires completing the test weapons he has available every week. For instance, he could have sniper to test the requires Crucible kills or an auto rifle the requires killing Cabal. To rank up faster I would recommend completing the nightfall before hand for bonus reputation. Edit: Every Wednesday the gunsmith has new rolls for orders which is known as Armsday!

So when u order them u just hold them and hope for a good roll?

Yup