The best way to do it is Print out a picture of the card (found at
www.cardkingdom.com or TMD's [/card] base), and then slip it in the sleeve over a bad card. Preferably, put the bad card the same way as you would a good card (face out) because you can see the back faintly through some plastics. Also, adhering the printout is usually bad news, as the card gets thick and you can get called for cheating. I hope this helps, as a vet of proxy tourneys I've done this a decent amount. That, is the BEST way to do it.
I guess an
acceptable way would be to write the card on a chaff card and throw in in a sleeve, but this makes it harder to keep track of game state, and it can damage the plastics.