It is placed it in the path of the laser beam with the slit vertical and positioned so that the laser spot on the screen is as bright as possible.
Compare the size and shape of the spot that would appear on the screen with that seen without the slit.
The width of the slit is slowly decreased. Once diffraction effects become evident, what happens as the slit gets narrower?
Why is there no change in the pattern in the vertical direction? What do you suppose the pattern for a round hole would look like?