I've never had tank water... I don't know why it seems everyone on here has troubles lol!
There are two ways I find best (The shaking is horrible and usually scares your fish). The way I do it with my actual siphon (I use a siphon and an airline tubing) is to get water in it, make sure all the air is out, and raise it so the tubing doesn't bend down coming out of the tank (If it drops then goes back up like a u it wont work) Quickly submerge it back into the water when the water is about half way out of the tube with the open part up, but slanted a bit (Like this "/")
With my airline tubing a usually just suck for like .5 seconds then drop it into the bucket, no need to keep sucking.
I just always buy siphons that have the bulb thing on them so I don't have to shake or anything....I feel like shaking it scares the fish and I always worry about getting a mouth full of tank water if I act like it's a straw lol
I find that, as previously suggested, sucking on the end of the tube is the most efficient way to get suction in the siphon. I have two - one with a hand bulb and one without...the one without the bulb is WAY easier to get going than the one with I find. The only downside is the occasional mouthful of tank water, which is when I stop paying attention hahah.
I have 2 syphons because well.. one tank developed blue green algae and I don't need an outbreak.. one is the old style no bulb no shake style. I have ALWAYS started my syphon with the tubing in the bucket (or out a window) and dunk the fat end with the opening facing up.. once full lift higher than the tubing, when it's half empty dunk again (still facing up) and slowly rotate the opening down and start stabbing your gravel or sand. I have never caught a fish, shrimp or floating plant with this method.
If it's the one with a cylinder attached to tube, this is what i do.. it's super simple thankfully
1) cover the end of the tube with your thumb. Don't let go until the final step in this list.
2) ensure the tube is lower than the cylinder (so the water can actually make its way down)
3) dunk the cylinder into the tank: the goal is to scoop the water into the tube until the tube is filled with water and reaches the point where it joins with the cylinder.
4) once the tube is full of water, submerse the cylinder fully so that it too is filled with water and let go of your thumb.