Just get a bit of pipe long enough, decent diameter around 10mm internal.
Pop it down the filler neck and push it through till it stops. Then you wana hold the other end low down, and suck on the pipe until fuel almost reaches the end, then quickly take your mouth off it before you drink a load of fuel.
This will start a siphon and you just need to keep the end of the hose held low in a bucket and it will siphon it out. Best to do it with a clear piece of hose so you can see when the fuel nearly reaches the end thats in your mouth.
Alternatively you can buy a 'jiggle siphon' halfords sell them and this has a little ball bearing pump which starts the siphon motion by jiggling the pipe up and down. Although this can be a pain if the cars got quite a bendy filler neck.