I wish to rewrite demands like:

mydomain.com/this/is/a/test

to:

mydomain.com/url.php?i=this,is,a,test

How do i write a guide that may handle this?

Produce a .htaccess file using the below contents and put it inside your root web directory.

RewriteEngine On
Options +Followsymlinks

RewriteRule (.*) url.php?i=$1

That will produce a query string of this/is/a/test rather than your asked for comma-separated value that's nothing a fast string-replace function (like str_replace() in PHP) could not fix.