Eclipse users are usually familiar with the installation of Eclipse plugins. However, if this is all new to you, you can go here for the installation instructions.
You need to create a new file with the .apt extension, then open it with the APT Editor (which should be the default editor for .apt extension). APT Editor is a multi-page editor, which means it has 2 panes: one is a browser that displays the formatted version in HTML, the other is a text editor to write the content of the document. You can switch between View and Edit by using the tabs at the bottom of the editor's area. Note that when opening a non-empty file, APT Editor assumes that you want to consult the document and thus will display the View pane by default.