To which I would add some notes from recent experiments:Get some bottle wax (which is just a hard paraffin wax) and melt it carefully. I put the wax in a large stove enamel mug (normally used for camping expeditions) and place the mug in a saucepan to which I've added some sunflower oil to help transfer the heat.
Put the pan on the stove on a very low heat (trying to melt wax quickly can be dangerous) and let it slowly melt. Do not leave a wax pot unattended - it is just as much a fire hazard as a chip pan.
If the old wax is crumbling, chip it off, if it had a foil capsule, take it off completely. However, if it has an old wax capsule that is reasonably sound, you can simply put a new layer of wax on top.
I clean the top of the bottle first with a damp cloth, and then with a cloth soaked in acetone, which dries off very quickly, taking any traces of moisture with it.
When the wax is fully melted, plunge the neck of the bottle into the wax quickly and then withdraw it straight away. I then spin the bottle, gradually bringing it upright. This usually works OK, but sometimes air bubbles form at the top. If you want to dip the bottle a second time, stick the bottle in the fridge for half an hour so the first coat of wax fully hardens.
Sometimes I find it useful to spin the new capsule over a spirit burner to tidy up my handywork - sometimes I find myself cutting off the new wax, putting in back in the pot, and having another go...
1) You get a better result if the wax has only just passed it's melting point.
2) If you have to dip a second time, damping the first coat with acetone helps harden it, as the evaporation of the acetone chills the wax.
3) In addition to drying the neck after cleaning, the acetone degreases the glass and allows the wax to make a better seal, so avoid touching the glass between cleaning and dipping.
4) If the bottle has an old frail label, wrap it in cling film to protect the label as you spin the bottle after dipping. However, be careful, as it makes the bottle more slippery to handle.
5) If you get a small air bubble or two on the surface of the wax, bring a lighted match close to the surface before the wax has hardened. This pops the bubble. If you keep the match close by (you may have to light a second one) the resultant hole will usually seal itself.