You must have stock of card paper. You will have to cut it down in a rectangular fashion with the dimensions of 4 inches to 6 inches. Develop a contrast between the colour of the card stock and the colour scheme that is to be used as the theme in photos etc. You can always decide which colour will suit the best in which scenario. It is not much of a brainer.
Use photo-safe glue stick and paste it on the back of the photo which will be placed across the face of the card. You must glue the photo in a way that it makes sure the photo will not detach from the object. Press the photo smoothly so that it can find a decent placement at the card and can stick to it better. Do this with every card and photo.
The message on the card should be written on the left-side of the postcard. It can be a greeting message along with the brief description or the reason of the event being arranged. On the right-hand side, the address of the recipient should be written. A stamp should also be added to the upper right corner of the postcard and all cards should be mailed from your desired courier service.