The quick guide -
Register a domain name, such as at
www.namecheap.com (highly recommended), that will let you create subdomains and add A-records.
1. Register the domain you want, eg. rocks.com
2. Log in straight away to the registrar's control panel and add a subdomain, eg. linode.rocks.com. Note that this only applies where you use the registrar's nameservers, which is obviously easiest to do!
3. Add an "A record" to that subdomain which points to an IP used on your linode.
4. Wait the usual 24-48 hours for this information to propagate.
5. Log into your linode's control panel and choose "Reverse DNS Configuration"
6. Enter your complete v-host in the box (linode.rocks.com)
7. Assuming everything went ok, you will be asked if you want to use that v-host as a reverse of the IP you made the A record to. Click Yes! If it complains about incorrect lookups, you may need to wait some more time (failing that, post here).
8. Just wait another 6-7 hours and your v-host will be working! (although some IRC servers will reject it until they've caught up with the changes too!)
Hope that helps
Quik