Not much to it (as long as it works, ahem). Unpack/upload the distribution files
into a directory that can be accessed via the web. Then go to the corresponding URL.
A web-based installer will walk you through the rest.
If your website, say, http://www.mysite.com, is mapped to the directory /home/jdoe/www/, and you place the YesWiki distribution files into /home/jdoe/www/wiki/, you should go to http://www.mysite.com/wiki/.
IMPORTANT: for installing or upgrading YesWiki, do NOT access any of the files contained in the setup/ subdirectory. They're used by the web-based installer/updater, but you should really just access the YesWiki directory itself, and it will (or at least should) work perfectly.
Detailed instructions are available at http://yeswiki.net/wakka.php?wiki=DocumentationInstallation .