You can't construct the thin film itself with Quantum Espresso, but only calculate it's properties.
You first have to create the unit cell of the thin film with some other software. As an example here is a tutorial for creating a slab with a specific facet of Au, using VESTA.
Then you put the resulting unit cell in QE's input file, and you can run the calculation.
For thin layers it is important to note that you have to add a large-enough vacuum along the out-of-plane direction in order to avoid fictitious interactions of the layer with it's reciprocal images.
For a layer located on the XY plane this is really easy - you just manually enlarge the value of the Z direction of the unit cell.
You should check for convergence of the total energy if you want accurate results, or just try some 1-2 nm as a rule of thumb.