Obviously rigour, backing arguments with data, analysis of sources etc. are very important to the pros, but it would be a poor way to set out (which is the same as saying some of the nitpicking and dogmatism both methodological and factual you'll encounter from aficionados could be worth less than nothing to you -- take everything on your own terms). My advice is to find something you're interested in and just read -- there are any number of fascinating stories big and small scattered through time.

After that, try to get some big-picture perspectives on the great arcs of history -- eras, civilizations, philosophies, religions, social structures and so on. These broad ideas are often meaningless to pros, but it's essential to develop instincts and acquire broad contextual knowledge without which no amount of detail can ever be meaningful. Make your own observations and remember it's really all about human psychology -- someone else may be more knowledgeable about a given topic, but if you can see that their sense for human nature, social dynamics, politics or culture is flawed then feel free to stick to your own opinions. Test your ideas in different contexts and subject them to criticism and you'll correctly feel you're moving forward, plus it's more fun to that way. You'll learn much more trying to work things out independently (even if you later decide you've made foolish errors) than by swallowing some expert's opinions whole.

Only when you've completed this broad-brush portrait of the eras or cultures that interest you does it become necessary to look at particular questions in real depth. You can do this out of curiosity regarding a particular issue at an earlier point but shouldn't feel at all obliged to.

As you gradually mix in more detailed investigations, you can then begin to consider more 'professional' ideas about history as a subject: What is evidence? What makes an original source reliable or unreliable?

Your background in philosophy will be of immense help in unifying disparate phenomena regardless of where you look. It has always exerted a subtle but powerful effect on culture and society at all levels and scales, and will make your perspective valuable even to specialists.