And with regard to cutting the masks for a project like this:
Obviously it is easier if you start with masking tape that's the same width as your squares (i.e. 3mm width tape for 3mm width squares) as you only need to cut in one direction... But you could also start with a big piece of tape and just cross-cut a section of it into a lot of little squares...
And this is the bit that a lot of people seem to overlook: square up the cuts properly. Use a straight edge and some kind of guide for aligning it properly. Take the time to get the size of each square, and the angle of each cut, as uniform as you can.