If you have not tested before, select the project you wish to join, either through the Search a DNA Project box or by browsing through the list of projects on the Projects page. Then click on the title of the project in which you are interested. You will see information on the project. Below that you will either be able to fill out an order form or a Join Request.
If you have already tested at Family Tree DNA and want to join a project, you may do so on the Projects – Join page of your myFTDNA account.