To do this achievement you will need at least 1 vaal orb and 1 skill gem that can be turned into a vaal gem.
This site has a list of all the vaal gems.
https://pathofexile.gamepedia.com/Vaal_skillOnce you have a vaal orb and one of the skill gems all you need to do is use the vaal orb on the skill gem. Sadly, this could take a couple of tries. If it doesn't work you need to use a new skill gem. It took me 2 orbs and my friend 1 orb.
Thanks to Rashaka1 for pointing out that when you use the vaal orb and it does not succeed the item will become corrupted and cannot be modified further. You want the gem to say vaal and then the gem name.