Hello
First of all you need to create yourself an username, as it is mandatory if you want to hide your ip behind an undernet mask. To do so, browse
http://cservice.undernet.org/live , click "Register!", and follow the steps. You will require an ISP e-mail or any payed e-mail services for this. After you've completed the steps, login to the e-mail you've set for the user, and click the confirmation link that will be sent to you there. After you have confirmed your registration, you may login with your user at
http://cservice.undernet.org/live , and most likely change the password you were given in the e-mail, as it's formed out of random upper/lower case letters and numbers, which for me is hard to remember
And after doing so, you can start your irc client, connect to an undernet server and use /msg
X@channels.undernet.org login your-username your-password . After you have logged in, type /mode your-nickname +x
Then your ip will be masked, for example if you have the user superman, your host will be like this after it's been masked: @superman.users.undernet.org . Note that you first need to login with the user, then /mode nick +x.
Hope this helps,
Wolfyx