Hi,
It's pretty simple.
1. Take graphene from database into builder
2. In Bulk tools-->supercell
3. set A'=2A+B others remain unchanged.
4. Swap axes B-C and Y-Z
5. Repeat the cell in the plane as you desire.
6. Now u hav a rectangular sheet.
7. Now u can use device from bulk to create device
8. U can add spatial regions as gate dielectric and metal electrodes and set the voltages.
enjoy