Sometimes when you try to cut a square of fabric it seems like there is nothing you can do to make it come out right. The fabric slides under the pressure of the scissors when you try to cut it, and you end up with a piece of fabric you cannot use for anything except a rag. The best way to fix the shifting problem is to use your scissors to cut off anything that might cause the fabric to twist, like the selvage edge, and then cut the square using a rotary cutter.
Spread the fabric so it is smooth and flat on the cutting surface.
Trim the selvage edge off the fabric with the scissors. The selvage edge is normally woven tighter than the rest of the fabric so cutting it off will allow the fabric to spread out flat on the cutting table.
Lay the metal square on the fabric so the long outer edge of the metal square lines up 1/4 inch away from the freshly trimmed selvage edge.
Decide on the size you need your fabric square to be. Make a 1/8 inch mark with the tailor's chalk right beside the inch marker on both sides of the outer edge of the metal square. Do not remove the metal square until you have completed step 7.
Place the rotary cutter so it is against the edge of the metal square and the blade is touching the fabric. Hold the metal square firmly in place. Hold the cutter so the handle is at a 45 degree angle and roll it forward until you reach the tiny mark you made with tailor's chalk. Roll the blade across the other outer edge of the square to form the second side of the fabric square.
Lift the metal square straight up and turn it, lining up the inch markers on the outer edge of the square with the tiny marks from step 4 and set it back down. You will be able to recognize the fabric you are cutting as a square now, with 2 fabric edges and two edges formed by the metal square.
Roll the blade of the rotary cutter over the other two edges, right along the outer edge of the metal square to complete the fabric square.
Rotary cutters are very sharp, so you will need to be careful of your fingers and you will want to be careful so you do not ruin the surface of your table.
It is best to use a cutting mat made for rotary cutters, but they tend to be expensive. A layer of cardboard under the fabric will help to protect your table if you do not want to invest in a cutting mat.