Herefishy: When I have done it, I just leave the legs together in the band, put one end of the band in a clipboard, stretch a bit with the other end in a clip, and then run the marker perpendicular to the material, creating a barred effect. One could insure a bit of consistency by setting a marking so you know exactly how much to stretch the material methodically, and then use a ruler for more consistent barring, but I never bothered.
In the spirit of full disclosure, I did this for a time and then just ended up buying the barred legs, because it just was not worth my time. I much prefer the alternatives, silicone legs such as Sili Legs or bass skirt material, or Superfloss, both of which provide dramatically more movement in the water. I use rubber legs pretty infrequently these days.