usually with a rotary cutter, but sometimes with a scissors, very occasionally I'll tear it -
Seriously, if the whole piece is going to be cut into - say four inch squares -
I would probably tear off an 18 inch length and cut my pieces from that -
It is just so much easier to handle shorter lengths.
I make my first cut on the lengthwise grain - and allow the extra length for if the fabric had an off-grain beginning cut.
I also try to buy extra.
And to plan ahead before cutting if I only have a small piece available.
GREAT tips have been given here - such as drawing out the pieces on graph paper before cutting, etc.