Reading the relevant documentation
I know that there are users who think that reading the relevant documentation prior to installation is just a waste of time. If there is a setup installation program or a script, it's good enough for them. WRONG! Reading the relevant documentation can ultimately save you a lot of time and grief.
The APEX installation is very simple and straightforward, provided you are following all the guidelines in both the release notes and the installation guide. Both documentations are included in the APEX distribution file, so they are accessible right from the start.
The Installation process includes some pre-installation tasks, dedicated mainly to make sure that your database environment meets all the required conditions for a successful installation. It also includes some post-installation tasks, which are very important to complement the main installation and properly configure it.
In all my working days with the various versions of APEX, starting with HTML DB 1.6, I never had to deal with a faulty installation or upgrade of the tool mainly, as I believe, because I strictly followed the documentation instructions and guidelines.