The first steps are to choose the bottle, and to carefully measure its inside height (for the length the masts must not exceed). Then the hull is carved to fit the neck, and it should not fill more than 50% of the neck.
Then comes the enjoyable part of making a small model outside the bottle. Here is a picture of one of these just completed, and ready to go in the bottle. The trick is that the masts are hinged with a short piece of wire, and that the forward-leading stays are long threads that have to be taut to keep the masts upright. In the case of this brigantine, there are 6 threads, 5 leading through the bowsprit and one (the main stay) through the foremast Two - the fore stay and the fore top stay which become the main top stay and the main topgallant stay - run through both. The mainmast hinge can be just seen, but there is also one at the base of the foremast.
The following picture shows the model from the other side. All the topping lifts, yards, gaff and boom are designed with flexible connections to the mast, so they can swivel. The threads are taped or fixed to the temporary building board.
So what happens next? Let go the fastenings of the threads, labelling each carefully as to which it is. Fold the masts down, curl the sails as small as possible, and slide the ship in as shown in these photos. This is when you find out if it fits.
Next partially erect the masts but move the ship (inside the bottle) just out of the way of its seat. With a long wire tool smear glue on the hull seat (I use epoxy), then manoeuvre the hull on to it and press down. Leave for 24 hours. Really!! Don't get any glue on the inside of the neck, or especially the threads.
Now gently pull on each of the threads in turn until the masts are fully erected. Fix the ends of the threads outside the bottle. Then with another long wire tool, touch some glue to each thread where it goes through the bowsprit or mast. Leave for 24 hours again!
Nearly there. With a sliver of razor blade fixed at the end of a long wire tool cut off the surplus thread at the bowsprit etc. Don't cut anything else! Now insert and glue any fragile extras you left to this stage such as a coach-house, boats, etc. Paint around the hull if you have to. Put a fancy cork in the neck after everything is dry (1 week?) and make a turk's head as embellishment.
Well that's the essentials. I've left out the other 100 things you need to know to make one, but you get the general idea.