Open the .Deb ackage with your normal installer. Usually for this you just double click, opening it in the Ubuntu Software Center, but you can opt to use GDebi or run the terminal command sudo dpkg -i Downloads/devede.deb, replacing the dirctory and name with the real one.
In the Ubuntu Software Center, click Install and wait for it to finish. With GDebi click Install and wait for it to finish. With the terminal, type in your password (don't worry when the letters don't appear, they don't) then hit enter. Wait fo the commands to finish executing and return to the home state.