Please forgive my spelling mistakes on my first post. I was typing as I think and my main language is spanish. I didn't know I only have 20 minutes to do edits.
The second wasy is to determine how wide again, your border is going to be. Fold the fabric salvage to salvage, wrong sides together and make sure that the bottom forms a straight line. If it does, the fabric you purchased was cut properly, but that is rarely the case. Usually it forms a pocket that looks crocket. In this case shift you fabric left or right until if lays flat. Lay it on the cutting board as it is. This instructions are for right handed, lefties, please switch it the other way. Lay your longest ruler on the right edge of the fabric, usually your 24 x 6 inches. Use the bottom of your fabric to line your ruler straight and cut on the right side of the ruler. Now your edge is straight. Don't lift your fabric, turn the cutting board around. Now lay your ruler along the left edge of the fabric. If your ruler is six inches and you want more than that, add another ruler on the left side. For example: if you want an 8 inches border, put another ruler, on the 2 inch mark against your now left border, and put your longer ruler next to it.
Make sure they are straight and cut along the long ruler, on the right.
If when you open the fabric it looks like a smile, you did not have your fabric straight on the first step. If you feel more comfortable, cut a small strip first and make sure it is not smiling. Cut as many strips as you wish lining up against the left border, because if you don't lift the fabric it will remain straight.
Now you can cut the salvage across and sew your strips together to the desired lenght of the border as I explained before.
This methods don't work very well if you have directional fabrics, specially stripes, because is very rare that the design is printed perfectly on the grain of the fabric. Usually they shift from the grain and you have to decide if you want your fabric to be straight or the design to be straight.
This can make you think that you made a mistake, but in realty you didn't. Fabric manufacturers, specially cheap fabrics, don't keep in mind what you will be using them for. I save those fabrics for fusy cutting, when it really doesn't matter.
By the way, I see my borders as frames and rarely ever use heavily printed fabrics on the borders. I tend to use a solid color, or a fabric that looks like wood. I like to hang my quilts and think of them as framed art.
Please let me know if this is clear.
Maria