Well, I just posted my Letterpress card this morning and I've gotten a lot of questions about using those plates. While I'm not an expert, I'll just tell you what I've discovered, ok?
For my Big Shot the sandwich is: platform open to Tab 1, Letterpress plate, cardstock, cutting pad. Yes, only one cutting pad.
I had best results with Versamark ink or no ink at all. The Versamark looks very rich and deep on the darker colors of cardstock. When I used no ink, I sponged over it with the same color afterwards to bring out the impressions a bit more.
I also experienced the back side of the pressed cardstock being roughed up a bit. Don't know if it's my machine and cutting pad or just a by-product of using this type of plate.
When I inked the plate with Craft ink, I had nice results, but personally found the clean-up to be messy. The best way I found to clean my plate was simple dish soap and water. I've not tried it in the dishwasher yet.
Hopefully this has helped answer your questions. I have another card using the Letterpress plate that will post on Friday, so stay tuned for more samples!