How to participate ? You only have to precise the coins you want to exchange in your collection.
With whom to exchange ? On the page Exchanges, you will find the list of coins the other member want to exchange. If you are looking for a particuar coin, you will find the name of the members that want to exchange it at the bottom of the coin's page in the catalog.
And then ? Suggest the deal to the member of your choice through the courier service of the site. Then settle together on the details.