Taking a break from the wedding posts this week. So when you're sewing you should really get to know your fabric. With almost any project it matter where the grainlines are. So here's a wonderful (yes, my computer design skills are amazing!) diagram!
Another way to get a fabric on grain especially if the cut edge is crazy is to find the lowest point in the edge (like if it dips, find the part that goes farthest into the fabric.) From there you can pull the thread that is running parallel do the edge. With some fabrics, you can pull this thread out of the fabric creating a small gap. Then, using that gap as a guide, line it up with a ruler and cut it using a rotary cutter. If you're dealing with tighter woven fabrics you probably can't do that.
You can also snip a small cut into the fabric and then rip the fabric and the fabric will naturally rip along the same thread. This may distort your fabric.
You can cut something on the bias as well. In that case, you would still want to make sure your lengthwise and crosswise grains are where you want them to be so you can line up your bias grain correctly and most importantly cut it straight.