How to put on a torc

Basically, you fit the torc around your neck by applying torque to it.  First the definition:

A torc, also spelled torq or torque, is a large rigid or at least stiff neck ring in metal, made either as a single piece or from strands twisted together. The great majority are open at the front, although some had hook and ring closures and a few mortice and tenon locking catches to close them. Many seem designed for near-permanent wear and would have been difficult to remove. Torcs are found in the Scythian, Illyrian, Thracian, Celtic, and other cultures of the European Iron Age from around the 8th century BC to the 3rd century AD. For the Iron Age Celts the gold torc seems to have been a key object, identifying the wearer as a person of high rank, and many of the finest works of ancient Celtic art are torcs...

 The word comes from Latin torquis (or torques), from torqueo, "to twist", because of the twisted shape many of the rings have. Typically, neck-rings that open at the front when worn are called "torcs" and those that open at the back "collars".

Now this explanation from a curator at the British Museum:

Although this [illustration embedded at top] is one of the most famous examples, the form is typical: open at the front, with a flexible neck-ring made of coiled or twisted wires. This type of torc is put on and taken off by being bent out of shape. You can see that one of the terminals of this torc has been pulled slightly forward compared to the other one. This is the result of it being repeatedly pulled open to be slipped on...

This constant flexing caused a lot of stress to the metal neck rings of the torc. When you bend metal in this way, it tends to harden and become brittle. You may have experienced this first hand if you have ever wanted to break off a piece of wire for hanging a picture or working in the garden and did this by bending it back and forth until it broke. The same thing happened to some torcs. We have many examples of truly beautiful neck-rings which were worn to destruction – taken on and off so many times that they broke at the back. They have often been somewhat clumsily repaired, as in this case:

On the Continent, there are other types of torc, which sometimes have clever hidden clasps, hinges, or removable sections... The idea of a hinge was taken up in later British neck-rings found in south-western Britain. They have a discreet hinge at the back, and a clasp at the front that was hidden when the terminals were closed... But there are some torcs which I don’t think could be opened up...

For details on those latter versions and relevant photos, see the entry at the British Museum's blog.