Once your idea becomes fully mature, and you can call it an invention than first thought that comes in your mind is to get it protected. Many inventors will find a patent attorney or patent agent that they will work with as they take first step towards filling a patent application in hope of obtaining a patent. Others will try and represent themselves. Representing yourself can be a dangerous undertaking because drafting a patent application and engaging in patent prosecution (i.e., working the patent examiner to identify patentable subject matter) can be a lot more difficult than it otherwise seems. There are many pitfalls and archaic rules to navigate. But even if you are going to choose to work with a patent attorney or patent agent you should do whatever you can to understand the tasks associated with obtaining a patent. The more informed you are as an inventor the smoother the patent process will be and the resulting patent will almost certainly be much stronger. The more you know the better the information you can provide to your patent attorney or patent agent, which only makes for a better, stronger, more detailed and broader patent once issued.
One big mistake I see inventors make with alarming frequency relates to understanding the full scope of their invention. This is not to suggest that inventors do not understand their invention, quite to the contrary. In my experience when an inventor has an invention (and not just an idea) the inventor will know what they have done in great detail. Sometimes there is a challenge to convey everything to the patent attorney or patent agent, but the inventor has that information, it just needs to be liberated from the inventor and put on paper.

How is it possible to obtain a patent on something that you have not invented?

A good patent attorney, patent agent or inventor coach can help an inventor think outside the box and see that there could be much more to what the inventor believes they have invented. In a patent application you want to describe everything the inventor believes they have invented (or those things that the inventor has specifically invented and knows that will work). You also want to describe those variations, alterations, and different versions of the invention that fall within the universe of the invention created by the inventor. This is precisely what is meant by “describing the full glory of the invention.”
Given that our scientific understanding is that perpetual motion machines cannot exist, and given that inventors frequently file patent applications claiming perpetual motion machines, the Patent Office does require a working prototype, which will be tested. So if you do not claim a perpetual motion machine the patent examiner will never ask you for a prototype. Therefore, all you need to do is define the invention in writing, through the use of text and illustrations, so that someone of skill in the relevant technical or scientific field would be able to understand the scope of your invention, understand how to make and use the invention, and understand what, if any, preferences you have relative to what you are claiming as your invention. Thus, explaining your invention on paper is all that is required, so you don’t have to limit yourself to what you have specifically invented, you can describe various combinations, permutations and alternatives that you have not yet built or tested.