"When I tell you this story, remember it may change." - Sherman Alexei, Captivity
When creating a completely original history, many people have a tendency to make either an ideal or a completely un-ideal world or society. One of the first things to realize is that you're working with an entire world. In creating its history, you are permitted to include multiple civilizations, each of which can be classified as ideal, un-ideal, or something in between. Or, you can make a single, unified civilization. Again, this civilization can change with time in whether it was good, bad, or just something else, as could any civilization in the first scenario. The possibilities are endless.
You can base it in earth's history, and make allusions to earth humans being in the far past, almost forgotten (Rene Barjavel's novel The Ice People is a great example of this). Or, it can be totally original. When you start, however, it is usually a good idea to have at least some conceptions of what you want to create, so that you can get a rather decent Conhistory going.
Remember, you may rewrite any part of this at any time, as none of it has to be true: remember also that the way history is told has much to do with who is telling it. Winston Churchill said "History is written by the victors", but the conquered people also have their story to tell.
Be ready to be challenged if you include controversial subjects, such as an idealized world that uses a certain political structure. Well, let us begin constructing your "ConHistory".
Before You Write
Prepare for Writing
It is recommendable to get a notebook if you want to do this by hand. If you will do it on the computer, you may benefit from using multiple files to write separate parts, but you may want to still have them open to gaze over. If you want to make this very long and drawn out, get ready for a lot of writing.
Your Civilization(s) in the Present
You don't need to write this for your Conhistory, but it helps to know where your history will lead up (or down) to in the end. If you skip this, you are being brave in not knowing where your conhistory will lead, or you know how it will lead, if only vaguely. If you have made a conlang of any sort, use things from it to help to shape what this world is now like. Consider politics, economics, sciences, art, and geography. Don't be afraid of verisimilitude in your ideal society, or glimmers of hope in your corrupt society. Don't be afraid to have something in the middle. Consider all that you know you want this to be. You can be as creative as you want here, and you can revise it alongside your Conhistory.
Before your Civilization(s) (Past)
The rest of this wikibook covers most of this information. It is common for a Conhistory to reflect Earth's history, since we know that Earth's history is possible because it happened. You may have ideas about alternative progressions of technology and the like, but it is not a bad idea to start with the people in a small-scale state, being nomadic hunters, or maybe what they were before that. Often, you could include some kind of creation story/myth here, if you are going back that far (Tolkien would say that some kind of creation myth is imperative). Anything goes; you can start in the distant past, or later on in your conhistory, progressing through various ideas and events that lead up to your current society.