I studies Physics in the days when a Physicist made their own equipment; a mandatory subject was scientific glass blowing.
There are various kinds of glass. The main type used for ordinary bottles and such is soda glass. This answer will focus on this glass.
Soda glass can be worked (from memory) at around 600°C. So if you had some glass rods or tubes, you could heat it to 600°C, and just join them together. More on that later.
But to completely reuse the glass, it needs to be properly melted. For soda glass that means 1040-1090°C.
The liquid glass could then be cast, but a cleaner result is achieved by blowing or using various shaping tools. To blow the melted glass, it needs to cool to a working temperature of around 900-1000°C.
As mentioned above, the glass can be worked at lower temperatures.
After any heating, the glass item should be allowed to cool slowly; this is called annealing. It's purpose is to relax the strains in the material that are caused when one part has been heated more than the rest. Improper annealing usually results in the item cracking.
Usually, one would use three furnaces. One for the melt, one for reheating work between steps, and an annealing furnace where the temperature is gradually reduced.
Since the material is at a high temperature, safety precautions must be taken. Firstly, the worker must understand the treatment of burns, and the use of protective equipment. One learns quickly to never handle the work with bare hands until it is quite cool. Leather gloves, steel tongs, shears and blowtube, and a steel work surface are all required.
Since most small scale glass work is carried out using gas torches, the use of a solar concentrator would require an additional learning curve.
So, to answer the question: if you don't have the equipment and knowledge - quite difficult. But IMHO certainly possible.
Useful Wikipedia references are the physical properties of glass page and the Glassblowing page.