Now that you have your straight of grain, cutting your fabric will be a piece of cake--almost. There are a few tips I'd like to share about actually cutting your fabric.
Be sure your fabric has some stiffness to it and that you have ironed out all the kinks and creases. Overtime, you will develop a sense of what you like in terms of how stiff you want your fabric.
Using the right tools can make all the difference. Using a rotary cutter is a great option for cutting fabric. Some traditionalists, though, continue to use scissors. The choice is yours to make. I have used both tools and use one or the other depending on what I am cutting. Sometimes, cutting long strips of fabric can be difficult with a rotary cutter. I have been known to draw a line on the fabric and then use my scissors to cut those long strips. I use a rotary cutter for the sub-cuts.
When cutting, make sure your blades are sharp. This is true for scissors and for rotary blades. Dull blades pull on fabric and can make your cuts jagged, off grain, and smaller. Sharp blades glide across the fabric with little resistance and make for a much more accurate result.
Rulers are essential, and accurate rulers are paramount. Not all rulers are equal. Some are more accurate than others. I have learned this lesson the hard way. I was using one company's rulers because I liked a certain feature of it. But my pieces were never the size they were supposed to be no matter how careful I was. It finally dawned on me that the ruler might be off, and indeed it was. I abandoned those rulers and switched to Creative Grids. Creative Grid rulers are extremely accurate, and my piecing is so much more accurate since I made the switch.
You might not think that the cutting mat can make a difference, but it can. Not all cutting mats are manufactured the same way. First, mats are not known for their measurement accuracy. If you use your mat when measuring, do not switch from using your cutting mat to using a ruler when cutting in a single project. Use one or the other. The difference in accuracy may be minimal, but over the size of the project, the difference can add up. It can also make matching seams difficult. Check the accuracy of the cutting mat's measurements before deciding to use it as your measuring tool.
The surface of your cutting mat can make a big difference. A popular brand of mat sold at big box stores has a very slick surface. This makes it easy for your fabric to slide while you are cutting. Avoid this kind of mat. Better quality mats have a rough texture on the surface which helps grab the fabric and makes it harder for it to slip around. Self healing mats are good to use. When a mat doesn't heal, the cutting scores become a way for your blade to get caught up and move off course. The self-healing mats have less drastic scoring.
When using a ruler, there are thick lines and thin lines. When using a thick line, use the outside of the line (or the left side of the line). Do this consistently. With the thin lines, the line should be right on the edge of the fabric, one on top of the other.
Hold the ruler down firmly and cut along the edge of the ruler. Try not to veer away from the ruler, especially at the end. We tend to veer off to the right making those larger pieces. Train yourself to keep the blade straight up against the ruler the whole way through your cut.
Go slow and steady through this stage. Cutting incorrectly and inaccurately can add levels of frustration that can be avoided by just slowing down and being attentive to what you are doing. Some find this part tedious, and it certainly can be. But this is also the place from whence all subsequent issues arise. This is probably the most important stage in the quilt making process.
Following these tips will hopefully help you cut more accurate pieces and enjoy the process of creating a quilt.