Tip:
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First, create your bottleneck area, the height of the bottleneck is really up
to you.
Most important part is the diameter of the neck.
As it would depend on the size
of the plastic or metal palm which you be using to screw onto the bottle.
Now that the diameter is correct, we will create the spiral groove.
You will need to try and practice
before you will get this right. Start from the top, hold your tool steady
and moved gradually downwards.
We're going to do it three times to create a deep groove
so to get this right make sure that your hands
are steady.