See original thread here:
Pen Ink on Leather Seats
I have not tried this on leather, but it works everywhere else. First.. the fresher the ink, the better - of course.
Hit it with either aerosol hairspray, or Lysol spray (any kind) - they all seem to have the same magic ingredient. Just a quick spritz. Now wait 5 or 10 seconds and watch it. If the ink is fresh and on a hard surface, you will see a sudden reaction - the ink will kind of burst open in a little haze. Wipe it off immediately. This works best on things like non-porous countertops and smooth plastics. I'm interested to know if this happens with leather, too.
The older the ink, the more set it will be and may not be completely removed. This will work on cloth too.. scrape your shirt with your pen, spray one of these aerosols on it continuously until it flushes it out, then run it under cold water to get the residue out.
You have to play around with different materials to see what works best.
I've used interior cleaners like Armor All on my van'* vinyl. I spray it on and leave it for about five seconds to get stains out - trouble is, it seems to melt the vinyl off, taking the stain with it. This stuff will quickly smooth out the original graining pattern molded into your interior panels.