Page 1 is the inside left-hand page. Usually, this page is left blank. So, in your document, you can press Ctrl+Enter to create a hard page break and leave that page blank.
Page 2 is the inside right-hand page. This page is where you put your sappy message — maybe a graphic.
Page 3 is the outside "back" cover. This page can be blank, or you can put at the bottom some tiny text boasting of your word-processing prowess or that the card would have cost $3.95 ($5.95 Canadian) had you bought it at a fancy greeting card store.
Page 4 ends up being the cover for the greeting card. Put a graphic and/or flowery text here.