Step 1: Measure how much you need (measure twice, cut only once is a really good rule to stick to) and make a small snip on the edge of your fabric where you want to cut.
Step 2: Ease the end of one thread free from the fabric. Pull on this thread a little until your fabric begins to gather, slide the gathers along the fabric away from you and keep pulling gently.
Step 3: Keep pulling on the thread and slide the gathers to the other side of your fabric. Don't worry if the thread snaps, you can just pull the end out again and carry on where you left off.
When you get to the end and the thread is pulled free, smooth out the gathers in the fabric by gently pulling it straight. You are left with a line where the thread is missing in your fabric which is really easy to follow and guaranteed to be straight.
Here's a close up of the line you can cut along
I hope this quick lesson helps those who are nervous about cutting their fabric to size - it works on any evenweave or linen.