Prepping and cutting your fabric is perhaps one of the most important steps in the quilt making process. If your pieces are not cut correctly, small errors in each piece's size multiplied over the size of the entire quilt can add up to a major difference in the size of your quilt compared with the pattern dimensions. While the final size of your quilt isn't a big deal like it is in garment making, it is indicative of inexact cutting.
Truth be told, this is the area in which I struggle most, so it is the skill that I have worked on the most. As a result, my cutting skills have improved substantially, and I have developed some techniques along the way that seem to work.
Iron your fabric and use a starch or sizing product to stiffen your fabric. Some award-winning quilters actually dip their fabric in vats of starch to saturate them. They hang the fabric on drying racks, and, once dry, iron them. This process makes the fabric extremely stiff. I take a less severe approach, but I do use a fair amount of starch to stiffen the fabric so it doesn't shift as I cut it.
Finding the straight of grain is another key component. When fabric is on the straight of grain it is at its strongest. As cuts shift away from the straight of grain, the fabric loses its integrity and is more prone to stretching out of shape.
So what is cutting on the straight of grain? It is cutting on either the warp or weft threads that are woven together to make fabric. Fabric has two grains--cross grain and length grain. Stay on either grain when cutting. Cutting on the bias is the weakest part of the fabric. It is extremely stretchy.
There are a few methods for finding straight of grain. One is to tear the fabric. As the tear continues, it will find its grain. This method usually requires that you purchase more fabric. Fabric is not printed on the grain, and you can lose an inch or more of fabric when tearing it.
A second method is to pull either a warp or weft thread at a point on the edge when you would get a full length of the thread. It will give you a line to begin cutting. Be forewarned: this is a frustrating method because the thread keeps breaking as you try to ease through the length of the fabric.
A third method is to fold your fabric selvage to selvage until it hangs straight. Don't worry about what the sides or selvages are doing. Focus on the fold and getting a nice straight line. Line the fold on a line on your cutting mat and cut off the sides.
A fourth way is one that I would reserve for very special projects or ones you plan to enter into a contest. This is a time consuming method but it is the most accurate method. Take your fabric and start pulling off your weft thread. As you do, you will get varying lengths of thread. Once you reach the point where the thread is pulled the entire width of the fabric, you have found the grain. This is similar to the second method but is actually easier. Sometimes pulling the thread inside the edge can be difficult and the thread breaks easily. This is a less frustrating method, but can be time consuming.
Finding the fabric's grain gives you the strongest fabric pieces that will withstand wear and tear and washings.