It varies from host to host, we can't provide exact installation procedures that will work for everyone. However, we do provide a setup tool that helps you make educated guesses at where all the settings are.
The easiest way that should work for most people is to upload the web files someplace in your document root (probably public_html) and your payvex files in your HOME directory, or at least not under public_html.
public_html/web/admin/setup.php payvex/conf.php(Notice how payvex/ is not in public_html/)
Then, just run setup.php and see what it tells you, it'll try to guess your home directory and give you some guidance on what the settings should be. Some ftp programs botch the directory names, so, setup.php will attempt to calculate the pathname from the servers perspective, this can help you trace down where a given path ought to be.
Setup.php will also help you create an ini file with your database settings that you can use. It doesn't save it to the server because in most cases, payvex won't have write access to its own database configuration. (Once this is set, you can use the control panel for the other settings)
Some people can't use home directories because their ISP's disable that functionality, for those hosts it is possible to set an environment variable instead. The setup.php script will help you do this.
setup.php is pretty colorful in the way it expresses things, it shows your current PHP information, as well as converting XML files into view information, so you can adjust things like error messages and simply re-run the setup tool.
PayVeX ships with more instructions, should you need them. (however, the setup.php will probably tell you everything you need to do)