Hi there,
Do you have the SU Scoring Plate? Want to "add" to it's use? How about getting the Diagonal Scoring Plate? All it does is fits right over top of the original scoring plate (& it is only $15.95). You can create your own envelopes with designer series paper (one side plain and one side patterned so it looks like you created something really cool!)
Check out the SU link:
http://www.stampinup.com/ca/enc/20701.asp
Also, check out this link from Youtube & Pattystamps (another SU demo):
http://www.youtube.com/watch?v=sQFK125dGNY
Enjoy!