The first thing you have to do is to make sure that no prior claim exists on the particular TLD in which you are interested. To do this, perform a quick search of the internet for it.

If the TLD is unclaimed, the next thing is to decide whether or not you want it to be an OpenNIC TLD. OpenNIC has some very specific and strict rules which regulate the way OpenNIC TLDs may be operated. Most importantly is the universal requirement that all OpenNIC TLDs must be governed democratically, with each domain registrant receiving a single vote. A vote of the "membership" of a TLD is always binding. Therefore, an OpenNIC TLD does not belong to any one person or group, but rather to the community of its users. Also, the TLD manager must provide a dedicated always-on Tier 1 OpenNIC nameserver, as well as manage domain registration and maintenance.

If you are unable or unwilling to abide by these rules, then OpenNIC is not for you.

If you are willing to...
manage a TLD which will be governed by its users,
manage a TLD which will always be subject to any binding vote of the entire OpenNIC membership,
provide a Tier 1 nameserver,
coordinate registry functions,
make all zone data for your TLD freely available

...then you may wish to propose your TLD for inclusion in the OpenNIC root.

To do so you must write a description of the TLD, and of its proposed use, and any restrictions on secondary domain ownership, use, transfer and content. It is probably a good idea to specifically address how disputes will be resolved, as well as the role of trademark/copyright in deciding disputes, since these issues are of concern to the OpenNIC membership. This document, which need be no more than a page or two, but can be longer, is called a "charter". Submit your charter to the OpenNIC Discuss mailing list (see MailingLists). Your proposal will be debated at length, and, if it is interesting to enough of the OpenNIC membership it will be brought to a vote. If the result is positive, then your TLD will be included in the OpenNIC root, and your will become an OpenNIC TLD manager. Good luck!

On a final note, we recommend that before you go to the trouble of preparing a formal TLD charter, that you post an informal query to the mailing list. You will probably get quite a bit of feedback, which will help in the preparation of your charter.

Here are the specific requirements for a TLD, and the steps to creating one:

each TLD must run at least one Tier 1 server which mirrors the root data of all the OpenNIC TLDs
each TLD must have a web site at http://www.opennic.[TLD] for registration and administrative information
each TLD must have the following email addresses:
- hostmaster@opennic.[TLD]
- abuse@opennic.[TLD]
- webmaster@opennic.[TLD]
unless specified in the TLD description and authorized by vote of the OpenNIC membership, the TLD governance must be democratic and allow one (and only one) vote to each real person holding a domain registration in that TLD

Here are the steps for creating a new OpenNIC TLD:
join OpenNIC - at the moment this is accomplished by subscribing to the MailingLists Discussion email list
form an admin team - gather a few other administrators to design and maintain the resources for the new TLD
propose the TLD - choose a TLD name and write a description of how it will be used; write a Charter for the new domain; post these to the discussion list
call for a vote - at any point in the list's discussion of the proposal, any member may call for a vote on the proposal; new TLDs will be established by collecting more than 50% of the votes cast (so if you don't like it, you'd better vote against it ...)
launch - make your DNS server(s) publicly available and start accepting registrations