I placed my bottles in hot - not boiling water - to make them easier to work with for joining the two top sections together. I am not going to say it's easy, as it requires a bit of fiddling around, but after a while you will get the hang of it.
Where you cut 5mm below the decorative section is where you want to have two top sections joined together. The hot water also helps to 'pop' the bottles back into shape if they become dented. You can also use the handle of a wooden spoon to poke inside and pop dents out.