First of all there is no etched-in-stone way to become a sovereign state. Some follow military ways e.g. Bangladesh seceding from Pakistan and becoming an independent country. Some follow political ways e.g. Pakistan and India carved out of united British India. There is no way to get the entire international community on board as interests of all states vary. They rarely agree on anything anyways.
You could say that a state is considered independent if it is considered a person in international law.
There are however two theories which can be considered as "How to be Independent" guides. They are:
- Constitutive Theory
- Declarative Theory
This theory concludes that a regional entity is considered an independent state or a person in international law if and only if it is recognized as such by other sovereign entities.
In 1815, Congress of Vienna Final Act acknowledged existence of only 39 sovereign states in Europe. It further outlined the process by which other states who claimed to be sovereign could be acknowledged as such. The process was nominally acknowledgement of their independence by the 39-club which in practice meant that a new state would be recognized as independent if and only if the Great Powers France, Russia, Great Britain, Prussia and Austria recognized them as such.
Lassa Openheim in 1921 however provided a realistic view. He said that even though there is no requirement by the international law for a state to be recognized by others for it to exist or its independence to exist, the Law simply refuses to note their independence. They may be de-facto independent however de-jure, they shall not be treated as such unless other nations recognize them.
Of course recognizing a nation is also a subjective thing, subjective to a number of factors. Chief among those are the interests of pre-existing states. If the new state serves interests of existing states, they might recognize it, if they don't, they might choose not to. Hypothetically consider Kurdistan. If an independent Kurdistan is established today, Israel would be inclined towards recognizing them as an independent Kurdistan is beneficial for Israel. It provides them a friendly partner in the region and it also weakens their regional rivals such as Iran whose own Kurdish region could be stirred to follow the same suite. Iran on the other hand, will oppose formation of Kurdistan for the exact same reasons.
Declarative theory is more clearer than the constitutive one. It lays out the following criteria for a state to be considered independent:
- It has a defined territory.
- It has a permanent population.
- It has a government.
- It has a capacity to enter into relations with other states.
The most important point is, if these four points are met, a territory is considered an independent state even if it is not recognized by any existing independent states.
See the Article 3 of Convention on Rights and duties of states or Montevideo Convention which not only explicitly states that the independence is not bound to recognition by others, it also grants new states the right to defend themselves. So if, let's say Catalonia starts a war against Spain, they would be well within their rights to do so as per Convention on Rights and Duties of States. This convention was adopted in 1933 and applies to all parties of International law and not just the signatories.
This was also the set of Laws used by Badinter Arbitration Committee to resolve the crisis of dissolution of Yugoslavia in 1991. So One could say that declarative theory is the one which is currently in practice.
In Practice however, one could assert that the only sure way to be seen as an independent state is to join the United Nations. The following is how you join the UN as a member state:
The procedure is briefly as follows:
- The State submits an application to the Secretary-General and a letter formally stating that it accepts the obligations under the
- The Security Council considers the application. Any recommendation for admission must receive the affirmative votes of 9 of the 15
members of the Council, provided that none of its five permanent
members — China, France, the Russian Federation, the United Kingdom of
Great Britain and Northern Ireland and the United States of America —
have voted against the application.
- If the Council recommends admission, the recommendation is presented to the General Assembly for consideration. A two-thirds
majority vote is necessary in the Assembly for admission of a new
- Membership becomes effective the date the resolution for admission is adopted.
So you need at least 9 friends in 15 members of the UNSC to pass the first step and make sure that none of the Permanent members veto the application, (Sorry Palestine, Chechnya, Taiwan, Brittany, Northern Ireland). If that's done (And this is the most difficult step as the General Assembly will mostly follow the suite), you need 2/3 majority in UNGA to gain admission.
But even by joining UN, you are in no way obligating the UN states to recognize you as independent. UN's recognition in itself provides you just a platform to communicate with the rest of the world. It doesn't guarantee the integrity and independence of your country for eternity. But that in itself means little because Montevideo Convention already accords you the right to defend your territory even if no state or UN recognizes you as independent. Not to mention, quite the contrary, UN may in fact take steps to hinder your independence. For example, UNSC resolution 541 which declared that Northern Cypriot declaration of Independence was legally invalid. Furthermore, UN could ask for advise from International Court of Justice as it did in Case of Kosovo.
If you take a look at List of States with limited recognition, you will notice there are UN members not recognized by other UN members, there are Non-UN members not recognized by a number UN members, there are non-UN members who are not recognized by any UN members etc. So there is no certain pattern.
Independent countries may choose their own ways to deal with your independence, e.g. the split among nations regarding Israel. Similarly independent countries may choose their own reactions to violation of your sovereignty i.e. support or opposition on the scale of verbal to in-field.
Standard IANAL disclaimer. I was hoping someone with good knowledge of International law could weigh in. But since the OP insisted, oh well.