1. Upload two POSCARs for your monolayer materials
2. Enter the maximum acceptable area of the resultant unit cell
3. Enter the maximum acceptable strain in the resultant unit cell
4. Hit submit
You can download the result POSCAR files using the Download POSCAR button.
Keep in mind that POSCAR 1 will be the top part of the heterostructure and POSCAR 2 will be the bottom part of the heterostructure.
The structures will be the same regardless unless either of the monolayers is asymmetric.