You need to start a syphon, and then let the water drain into a bucket though the free end of the hose (ie, the one that's not attached to the gravel cleaner; that end (obviously!) stays in the tank).
To start the syphon, you can either suck on the free end of the pipe (making sure it's lower than the tank end); with a bit of practice you can get your mouth away before you get a mouthful of tank water!
Ot you can coil the whole thing up and submerge it in the tank. Turn and rotate it until all the air is out, put your finger or thumb over the free and to stop any air getting in, then move that to your bucket
have your bucket down below your tank with your hose in it.. submerge the vacuum end and fill it with water.. bring it back up and allow some water to drain down the tube into the bucket.. BUT CATCH THE WATER BY PINCHING THE HOSE BEFORE ALL THE WATER YOU FILLED IT UP WITH DRAINS INTO THE BUCKET .. put the cleaning end back down into the water, being sure to let all the bubbles out so theres' only water in the vac. .. once youve got all the bubbles out and the vacuum in position, unclench the hose and the siphon will start! works like a charm!!