Hi Stricktly, it really depends on where you are trying to fray them. If you are looking to fray only the bottoms of the jeans, then you'll need a sharp pair of scissors so that you can cut along at the base (not too much but just all the way around), do not cut the hemline/seam, as then the fray will continue. After you've done that, take your fingers and work the threads a little bit, I used to do this as a teenager, then I'd take the back end of a pencil eraser and rub the eraser along the edges.
After doing all of that, I then would put them in the washer on the extended cycle with liquid fabric softener (it helps soften the fibers), and allows it to break up. Then they go into the dryer and your should have successfully frayed your jeans. If they are cut offs, you can disregard the seam note above.
And if you are doing this for the knee area or other part of the leg area, do not make straight cuts into the legs, make shorter cuts and rub entire area with eraser, sometimes using a dryer sheet also helps, then rub the fabric together and proceed with the wash cycle. Hope this helps :)