The easy way is with Dopus, select the file and "Add Icon". this takes the default icon for your file.
You can change this icon later, by select with the right button, then in menu "icons/info", and drop over the image the icon you like. Don't forget to "Save"
Other way, is to open the drawer with your file, right button "window/show all", and then your file shows you the default icon for it. Select the file and with right button "Icons/Store", now your Icon is saved, and the .info created.
