once your fabrics are nicely pressed, you have two options for cutting. if you own a cutting mat, rotary cutter, and quilting rulers and are comfortable squaring off fabric this way, then square each fat quarter piece to approximately 18" x 21", or whatever similar size you can get out of your fat quarter.
if you don't have these cutting tools or don't know how to use them to square fabric, you can simply get some large paper (or tape some together) and use a ruler to draw yourself a pattern piece of a square that measures 18" x 21". pin your pattern piece to your squared fat quarter and carefully cut out the square. you won't get it as precise as using the rulers and rotary cutter, but it will be good enough for this blanket. remember, small imperfections are part of handmade's charm.
now you are ready to sew!