You need an air hose to do this. One end of the flexible airhose to the tank or a bucket of tank water, and the other end to the plastic bag. Suck on the airhose to start a siphon.
Now you need to control the rate of water coming down the airhose to the plastic bag. You could tie a knot on the airhose, adjust the tightness of the knot so it's dripping like a drop for every 2-4 seconds.
If you have ever go to a hospital where they do the IV, they have this little plastic thing (someone supply the proper name please) to fit over the airhose with a roller to control the drip rate. Some LFS carries these thing too, just ask them for a drip controller. Tying a knot on the airhose is easy, but it's harder to get a consistent drip rate.
When the plastic bag is almost full, or half an hour later, your fish should be able to put into the tank. Make sure the water in the bag DO NOT go back into the tank.