This can be a tricky, and potentially dangerous thing to do. I have done this the 'wrong' way many times, but never again, and I will not elaborate...
One obvious method includes disconnecting the hose, and letting fuel run into a container, but this would be particularly difficult on the 124, since the flexible line is so short. Also, you would need to have a big enough container, since shutting off the stream is tough.
There are many devices that can help get a siphon started, including the 'Wonder Pump', which seems to be sold in many places:
I saw this at the Marine store for just $17, which looks ideal for this purpose:
Gas-VacŪ Siphoning System at http://www.overtons.com
When I did mine, I used a suction pump like the one below, but as I mentioned in another thread, it has since broken apart after a minor 2-foot drop. It was good otherwise for a variety of purposes. I think it was about $40, but it seems to be $50 or more everywhere I look now. BTW, they do not recommend using it for gasoline, (so I will not either)...
I played with the idea of installing a shutoff valve so that it would be easier to service the filter the next time, but I did not have enough time to put it together.