Sorry I took so long getting back to your post, kev, but here goes.
The important thing to remember is that gravity makes water flow down hill. When it does that, it will pull additional water with it as long as there is no break (air) in the stream.
Hold the big end of the siphon in your right hand with the opening facing up. Put it into the tank and fill it with water. At this stage the opening is up and the hose under it is coming up (out of the water) and over the front of the tank and ending up in a bucket on the floor. Your left hand will be keeping track of this end so that it does not wiggle itself out of the bucket when you move the big end.
Slowly raise the big end until it is out of the water and the water in the hose is starting to flow over the top of the tank and down into the bucket. As this is happening, you will notice that the level of water in the big end is going down. Before it gets so low that it empties all the water into the hose, put it under the water to refill it. (If it all flows into the hose, you will get an air pocket which will cause the process to fail) Again, lift it up and once the water in the hose is over the top of the tank and flowing down hill, put the big end back under water. When the water starts flowing out the bottom of the hose, your siphon is started and it will continue to pull water out of the tank. You can then begin vacuuming your tank.
Once the siphon is started, if you have to interrupt it to empty your bucket, you could, while holding your finger over the opening, lift the little end out of the bucket and put it into the tank. When it is underwater, you will have both ends under water and the hose full of water. Close the lid on it to hold it in place. When you return, lift the little end out of the water while holding your finger over it and put it back in the bucket. The siphoning will continue as before.
This is one of those things that just takes practice. At first it will probably seem like the water flies out of the big end, but as you get used to it this will no longer be a problem. It helps if you don't lift the big end any higher than necessary, because the closer it is to the water, the faster you can get it back under.
Good luck and let me know how it works out for you.
