Pick a piece of 18th century jewelry and take one or two design
elements from it, ie- use of line, surface treatment, symmetry,
asymmetry, shapes, techniques, ect. Now design a piece that uses
these one or two elements but incorporate your own esthetic into
the piece as well. The idea is not to make a copy but to develop
your own sense of design, while borrowing ideas from the past.
Make a lot of sketches until you find a design that is pleasing.
This process will make achieving your goal of creating a
complicated piece less daunting, by working up to it, while
improving your design skills.