Installing ERW is not a very complicated operation. However, there are several details that must be set up correctly in the environment for ERW to work.
The first step toward using ERW's powerful editing forms is installing all PHP scripts coming with the distribution (the content of the php directory) somewhere where they can be served by Apache. These scripts contain the run-time environment that produces the editing forms starting from the definition files produced by ERtool.
More precisely, the file ERW.php and the directory ERW can be installed in any place where PHP searches for source files (i.e., /usr/share/php), whereas the remaining files must be placed in a directory served by Apache. A reasonable option is to install all files in a generic place (e.g., /usr/share/ERW/php) and then create symbolic links, but in this case you must be sure that the Apache FollowSymLinks option is on.
The second step is producing definition files with ERtool and making them accessible to the run-time environment. As a first try, you should place the definition files in a directory named defs located in the same directory of the scripts (it is the place where definition files are searched for by default). As a first access method, you can change the name of index.orig.php file to index.php: it produces a list of links to the pages that allow you to choose and edit elements of the database. However, you should devise some custom access page that fits your needs.