You need to go out & buy some postcards. It's a minimum of 4 postcards to an envelope/exchange.
If someone requests to have their postcards written on, then it mean, that you have a choice...........a personal greeting, or you could write an explanation of what the postcard is about.
If they are requested to be blank, then that means they want them in mint condition.
Does that help? It's the same as any other exchange, the only real differnce being that some people like their postcards written on & others prefer them to be in mint condition/blank.