Unless otherwise stated by your village leaders, travel is open between all villages. The requirements are simple; you merely need permission from your village commanders and a simple travel thread. Your village leader will dictate whether or not permission is needed for travel, and the basic requirements for permission will be decided by them. However, a specific set of requirements must be made for travel to be considered valid.
You must post a travel thread. This thread is created in the region you are leaving, and must be titled as:
"Thread Title [Travel (Region A - Region B)]"
This thread need to be a minimum of 100 words, and you must wait at least 1 hour from when the thread is posted before you may post in the area you have traveled to. Note, this is from the time when the last member of your party posts; if there is more than one person traveling in a thread you must wait for everyone to post before the 1 hour kicks in. The reason for this is to allow for Thread Interventions, by ANBU and otherwise.
If your travel is part of a defection from a village, you must wait for 24 hours before you may post in the new region, to allow for time for Hunter Ninja to intervene if they are going to.