I written a small music composition. I want to make it freely available and disclaim any copyright. How does one release a work into the public domain?
The relevant concept is dedicating a work to the public domain, that is, saying in the work something like "This work is dedicated to the public domain". I understand that this isn't entirely reliable in European civil law. The preferred alternative is to license it to the public. However, you have to decide how "free" you want the work to be made. The normal state of affairs, where you do nothing and just rely on copyright law, is that you have the sole right to allow copies to be made and derivative works to be created. Thus if someone were to make a derivative work based on your composition, they would need your permission: but then they would have the right what they created (such as a translation). If you just abandon your property right to the work, you impose no obligation on others, and a person can freely create a translation (which is now their property). If you execute the right public license, you can allow people to use your work as long as they include that license in their versions.
A fairly common public licensing scheme is the Creative Commons licenses. That article gives a decent summary of relevant rights and how particular licenses correspond to configurations of permissions. I would say that the most difficult thing to do is to figure out what you don't want to happen, and pick a license that matches that interest.