You will need...
1. Blue bottle with screw top.
2. Large bucket (I used an old bird-feed tub)
3. Measuring jug
4. Water
5. 1x Studio flash head
6. 1X flash gun
6. A towel
7. A day when the wife is out
8, A reflector
9. Roll of plastic insulating tape
10. Some kind of boom arrangement (I used a reflector boom on a light stand)
11. Black back-cloth (for some of the shots)
Here's what I did.
1. Cut about 15 inches of tape and fold in half (sticky side to sticky side) to make a strap for the bottle. Tape the strap to the base of the bottle (see image).
2. Fill with water and screw cap on. Then hang bottle on boom.
3. Set up your lights and do some test shots. Get this right before releasing water as it's time consuming to refill.
4. Tip bottle up while still on boom and undo cap. Place the palm of your hand over opening and let bottle relax down to resting position with your hand still on opening.
5. Release hand, pick up camera, and shoot. You will probably only get one shot per refill, so make it count. If you have an assistant, get them to release the water, and you may get two shots.
6. For subsequent refills, pour from bucket into measuring jug, and from there back into the bottle.
7. In Photoshop, rotate the shot to suit, and edit in normal way.
I used 1/250s which is the limit of the 50D flash synch. Any faster and you get the shutter curtains showing. The studio flash was on the left of the bottle, and the flash gun was bouncing off a reflector on the right.
Here's the first one I did with a white back-drop.
And the second one (which is my favourite)
Here's a 100% detail blowup...