The file 'Bed Levelling Squares (150x150mm) 3D Printer Model' is (stl) file type, size is 742.0B.
You can use these squares to easily level your bed. Print them out and then adjust each corner individually. If the surface is rough, lower that corner and if it's not sticking at all or you see gaps between the lines, then raise that corner slightly. Keep going until all of the squares are smooth and you should get a perfect first layer.
This version is intended for bed sizes of 150x150mm. Here are links for other bed sizes:
I've also made a video explaining the bed levelling process and you can watch it here:
15x15.stl | 3.0KB |