You can avoid vise-marks if you use a plastic opening tool, I was able to do it pretty easily. First I applied pressure with my fingernail which created a slight rift, at which point I realized I had a plastic opening tool, so I was able to use that to pry the halves apart, and hold the small opening with my fingernail while I ran the plastic opening tool along the rest of the seam.
You can avoid vise-marks if you use a plastic opening tool, I was able to do it pretty easily. First I applied pressure with my fingernail which created a slight rift, at which point I realized I had a plastic opening tool, so I was able to use that to pry the halves apart, and hold the small opening with my fingernail while I ran the plastic opening tool along the rest of the seam.