Most of the time if you have a length of fabric, it won't need to be more than 3 yards (unless you are making a huge king quilt). So the most important step is to get your selvage edges perfectly lined up and straight, one on top of the other, and then press the fabric.
Then you fold as many times as you need to in order to get the fabric under your ruler. If you are just using a regular fabric with no particular stripe that you want to follow, you can get border cuts of up to 10" out of a width of fabric. Most borders are 6" or less usually.
You can use a 6" x 24" ruler, but you will need a large 60 mm (not 45 mm) blade in your cutter in order to go through all the layers, and it must be a sharp blade with no nicks.
I prefer to use a June Taylor Shape Cut Plus for this process. It is a ruler with laser cut groves for your blade to glide in and it does not slip like a regular ruler may. I think if you look at their web site, you can see how it would be used.
http://www.junetailor.com/shapecutvido.htm
If I were cutting a border stripe, however, i would definitely use a scissor. As tedious as that is, you really need to make sure you get your 1/4" seam allowance added on to the stripe you want. I suppose you could use a rotary cutter, but it would have to be single thickness of fabric, and done carefully.