I am new to the board game design world but so far I've seen a trend of approaching board game design by NOT writing any rulebook until late in playtesting.
In my opinion this makes no sense...
The first thing I do when I have a new idea for a board game (a solid board game idea, not just fleeting 'oh this could be a cool mechanic' type of idea, but a solid board game idea) is create a blank Google Docs document and immediately begin writing the rules (sometimes I even *GASP* spend a good 15 to 20 minutes thinking up the name of my game). I have gotten to the point where I will have pre-titled sections such as "Set-Up, Turn Order, End Game, Additional Rules" etc.
I try and flesh out as much as the rulebook as I can, sometimes this is not a lot, sometimes it is almost the entire first version of the game, but most importantly is now I have a live document that I can refer to on my phone, and edit on the fly.
The rulebook IS NOT SET IN STONE, it's constantly being edited, and as I just stated, can even be done so on my mobile device during the middle of a playtest.
The reason why I think it's important to have a rulebook first is because it allows you to have a live document you can refer to on rulings during playtesting. Rule doesn't work? Change it in the document live. That way, you know the next playtest you will be playing with the updated rule. You can also track your changes in the history of the document, this helps avoid problems of implementing old rules you changed in the first place (unless those old rules turned out to be better)
Also, added bonus: by the time your game is ready to take to kickstarter or to publishers, you already have your rulebook written!
Anyway, this is just my opinion, and yes I used the Change my mind meme for my title... so, change my mind!
That does indeed change my mind a little bit, I hadn't thought about this. Maybe I'm just referring to my document as a rulebook but really it's more of a notes on the game. Ultimately the document will be used for what the Rulebook will be made from, so I guess that's why I title it a rulebook.