(My answer to this question at Quora.com)
The best technical writers are those who have a passion and a knack to explain things well in writing. If you meet those two criteria, you also need to learn the technical field in question inside-out, and somehow demonstrate or prove your knowledge and explaining ability to get hired.
In my case, I proactively created (and self-published) an instruction manual about a subject that I knew very well (a particular computer-aided-design application), which I found was more sought-after than my live training courses—so I created a new edition for each new version of the software, and these sold so well that they became the standard textbook in the field, and provided half my income. When I subsequently looked for a salaried position as a technical writer, I got the job at the interview as soon as I plonked copies of my books on the table…
As for where you work—if you’re working for someone else, you really need to work at their premises, because it requires working closely with the technical teams to see the issues and really understand the product. If that company is in Italy, well and good—otherwise, you will have to relocate.