I was able to make very good counterfeits in the late 90s (I actually never sold any, just a bunch of friends had a competition to see who could make the best fake):
1) Erase the surface of a real MtG card (using one of those erasers that were roughly the consistency of thick gelatin it could be done in about an hour).
2) Scan the card to be duplicated
3) Split out the black channel using photoshop
4) Pattern fill the missing black channel in photoshop. Hand fixup any obvious bad details
5) Print the color on a dye-sublimation printer
6) Print the black on a laser printer
7) Put a thin plastic coating on top
Step #7 was the trickiest manual step, but could be omitted if the card was going to be in sleeves.
I was never able to get the right "feel" by making my own two-ply cardboard, but somewhat surprisingly the closest I got was my first attempt of two index cards rubber-cemented together.
Note also that Revised-edition cards lacked any full bleed, and the text backgrounds were quite simple. It might be harder today.