There is one big tip to making your own wedding cards: layer, layer, layer. Use a sturdy cardstock as a backing. This backing should be strong enough to hold a couple more layers of regular paper and the embellishments you want to add later. You don't want a thin cardstock because your invitation may turn out limp. The backing should also be as big as you want your finished invitation to be. Next is an optional lining_and this is where you can play with complementary colors and paper design. Cut the lining piece about half an inch smaller than the backing on each side. Glue the lining on top of the backing. Then add the piece of paper with the message. If you are including a ribbon that is "wrapped" around your invitation, you can use tape and tape the edges to the back of the piece of paper with the message. That way you don't have to have huge blotches of moisture from the glue you used for the ribbon, and tape doesn't have to dry. Once you have taped the ribbon back, you can go ahead and glue the last piece to your invitation. If you can see the ribbon through the piece of paper with the message, then try gluing the lining and the paper with the message first, tape the ribbon around both, and then gluing that whole piece onto the backing.
If you want the look of a traditional wedding invitation, but would like to implement your wedding colors, consider using one of your colors as the lining color for the envelopes. This is a subtle and attractive way to customize the appearance of your invitations. Other places to add accent colors are on envelope seals, monograms and motifs. Careful use of color can add drama or whimsy to your invitations without adding much to the price.