The process in summary is:
- Install a new Joomla latest release site
- install SP Upgrade on the new site
- insert in components options 1.5 DB credentials
- hit button transfer
- deal with extensions
Installing Joomla is very simple.
In summary in order to install Joomla on the same host as Joomla 1.5 follow below steps:
- Backup your 1.5 site and database
- Create a new MySQL database
- Download Joomla zip file from www.joomla.org
- Create a new directory on your root (e.g. called jnew). So if your root path is /home/public_html, you create directory on /home/public_html/jnew
- Upload and uncompressed the zip in folder jnew
- Go to url: www.yourdomain.com/jnew to continue with installation
In details read this: http://docs.joomla.org/Installing_Joomla!
When you are ready to go live all you have to do is to move all 1.5 folders and files from the root to a backup directory, and the j3x folder on the root.
For dealing with extensions read the corresponding section in this site.
Another good guide for the migration is this: http://www.ostraining.com/blog/joomla/sp-upgrade/