It's super messy and your hands will be colored for days, but it's also so much fun and so addicting, you'll want to keep going, trust me...have a pile of cut cards at the ready ;-)..
You'll need cardstock, normal or glossy works, use heavy paper because of moist content, I haven't tried watercolor paper, but it migh work also.
Shaving foam, it must be foam or cream not gel. The cheapest is fine, mine smells nice and so do my cards now ;-).
Re-inkers, i've used Distress, but any concentrated pigment will work, I've also done it with Talens ecoline and food coloring.
Lot of paper towels and a scraper, I use a doughscraper, but a card will also work.
In a shallow container, big enough for your cards spray a small layer of shavingfoam and spread it out. Place drops of re-inker across the surface and then using a brush, skewer or alike mix it in the foam.
I prefer staying within one colorfamily but you can go as bold as you want.
lay a piece of card on the foam and press the surface lightly, you do not want to submerse the paper. lift it off and scrape off excess foam. let dry....use it however you see fit.
i've used some of it for the Simon Say Stamp Monday Challenge.
to see what I made, click this