You may first import a bulk BiI3 layered structure from Crystallography Open Database as described at
https://docs.quantumwise.com/tutorials/builder_manual/builder_import/builder_import.html if you use ATK-2016, for example. In the newer version, this Database is available directly in Virtual NanoLab, see a button below the Viewer button, and the structure will be imported to the LabFloor, so that you will have to drag and drop the corresponding object from the LabFloor to the Stash in the Builder for further modifications.
After importing the bulk BiI3 layered structure, you have to delete all the monolayers in the unit cell of BiI3 but one. And you may also want to increase the lattice parameter value in the out-of-plane direction to ensure that there is enough vacuum between the image monolayers, which result from the periodic boundary conditions imposed by default in the out-of-plane direction. Note that the lattice parameter can be enlarged in the Builder Tools -> Lattice Parameters.
