Haven't used them for ages but used to put them in very hot water for about a minute, this helps to soften them, and then pull like crazy over bit rings!!
Ok, I'm getting to be an expert on his.. Firstly soak in hot water, then get two pieces of baling twine, loop through the bit guard and pull in opposite directions, stretching the hole over the bit. Get someone to pull one side if it proves to be a bit of a handful. You can skip the hot water in an emergency! I have to pull them over a continental and american gag at events and it's something I'm now rather deft at..
The way i did it was..... put in hot water for a bit.
Pass one side to my dad, and i have the other, pull it apart and pull the bit over the bit rings.
I usually just get two bits of bailer twine and 3 people, 2 people hold the bailer twine then lean back, the other person threads the bit through its the only way I have ever found works