I'm a lefty and I cut from the far right side of the fabric. I have to square it up from the left side of the fabric so the ruler is over the majority of the fabric, cut to square, then I flip it so the squared edge is on the far right. You will line up the squared edge of the fabric to the mark on the ruler at which you want to cut, i.e. 2 1/2". I hold the ruler down with my right hand and cut with the left. My cut 2 1/2" fabric will obviously be under the ruler and I continue going from right to left lining up the edge of the fabric with the mark on the ruler I want to cut at.
Ok, clear as mud I'm sure. LOL. I would check the internet too, there are many places to find lefty instructions. That's just how I do it.