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 is a single square (60x60mm) in the middle. If you want to calibrate the entire bed, you can find appropriate squares for different bed sizes here:
I've also made a video explaining the bed levelling process and you can watch it here:
Make sure to print at 0,2 layer height so that it will print only one layer.