All you have to do is buy a product containing the stamp card from the list of stores involved (some of them offer only limited products marked with the card logo, some others offer them in every product they sell) and mark them at the printing device located in all the 20 shops.
More info on the official site.
If you are managing to do the print rally all at once, probably you need many free slots where attach the cards in order to print them. I personally use the HUD slots, saving one for the AO but you can also wear them in every free slot of the avatar like many actually do.
Here are the unisex prizes available that I picked up:
Picnic camping car [SummerChoice!2009]
"anuenue. Hickory pants Choice2009
Uw.St Rouvel-Hair (CSR)
)) HANAUTA (( - Picnic Set "SummerChoice!2009"
Feather =F=LunchBox(SummerChoice!2009)
**DP**yumyum shy sheep/2009 choice
(iTuTu) Sunflower Cart // SummerChoice!2009
D-LAB GREEN TREE HOUSE-box
Vooner <
*Edelweiss* School Blazer Cure - Choice!
S.I.C_5_Jacket_Set_Choice! version_BOX
*LPP* Choice!2009 Gift / *LPP house PKG
*COCO*_SummerChoice2009_GIFT
=DeLa*= Boots "Tucson" Summer Choice09
