NO! NO! NO! You should never start a siphon with your mouth!

That's too disgusting to even think about. Icky. Ugh.
Look. The idea is that water flows downhill. If you remember that the rest is easy.
If you put the entire hose in the tank and let it fill up with water then take the small end out (while blocking the end with your finger) and put it in a bucket that is lower than the tank you will be siphoning from, when you release your finger it will start to flow.
Once you have that down pat, just fool around with the thing. Put the small end in the bucket, then scoop up water with the big end and lift it up high. Watch how it flows down and how long it takes to empty the big tube. If you put it back into the water before it the big tube is empty it will immediately fill back up. Lift it again. Once the hose is full of water to below the level of the tank, gravity will take over and continue to pull water from the tank if the big end is underwater. (I find that putting the big tube in the water with the opening pointing up works best.) At that point, the siphon is started and you can turn the big end around and start vacuming.
You may have top practice this a bit, but it is worth it. I start my siphon with just 2 dips of the big end.

You should never have to suck on it!
