Well, it depends on how fascist you want to really be.
You could just wait until fascism is enabled, and then let fascist rebels spawn, and then let them wander around the country until they take your capitol. If they hold it for a year, they reform the government.
If you are a HMs Monarchy, you could just appoint them as a political party. You are still a constitutional monarchy, but the fascists are now in power (not unlike asking Mussolini to form a government, actually).
You could also get them elected in any government that has elections. But you won't get enough POPs supporting the fascist party until things are really bad (missing cores, bad economy, artisans doing poorly, soldiers and officers doing poorly). Even then, you are still a democracy with a fascist party in power. They may just get booted out next election, especially if you reclaim your cores.