In short, like the very first things we were taught as infants, one learns by imitation and by trial and error. Reading, thinking, talking philosophy are all parts of the process. But none of these is a satisfactory substitute for the discipline of expressing your thoughts on paper. A student who has not yet produced his or her first piece of written work has simply not reached first base. By 'writing' one does not mean simply jotting down thoughts as they come into your head, though this too can be an initial part of the process.
Philosophical writing involves constructing an argument.
It is reflective and self-critical. Even when the writing flows, the words form an organised structure. For all the wide variations in style and presentation, the writings of philosophers possess a common architecture, which is none other than that of logic itself. Simply that the cogency of one's argument depends solely on reasoning and logic. The appeal to observations, or to the results of experiments or surveys, or to any other forms of recorded data has no place in a philosophical argument.