There are two ways to claim land.
The first is to conjure up some de jure rights (some way to justify legal ownership, such as 500 years ago your ancestors owned the land until someone illegally invaded or you inherited the title to the land from your great-great-great-* grandfather). It can be flimsy (or outright forged), but it needs to be recognized by other countries to be effective. Religious significance was once recognized as basis for de jure claims, but it is not anymore.
Then you have de facto control. If you can exert physical control of the territory, it's pretty much yours. There are companies you can pay who will do this. Reflex Responses might be one, but there are a number of them out there.
Of course having de jure rights without de facto control is pretty useless. It's a long game strategy. Spain one day hopes they will be able to take de facto control of the Gibraltar, but it won't be in the lifetime of anyone alive today.