I start with a broad outline of a design, but I inevitably change significant aspects as I go along. One of my bigger long-term projects right now is a word pr

I start with a broad outline of a design, but I inevitably change significant aspects as I go along. One of my bigger long-term projects right now is a word processor document templating engine. I've had to massively redesign a lot of the character set handling code, because the Real World of how, e.g. Microsoft Word determines the character set in current use is absolutely bonkers (there are situations where Word will interpret Windows-1252, which is pretty much ISO 8859-1 with extra characters in the 0x8_ and 0x9_ range, as Arabic for no reason other than a RTL marker). That's all by long way of saying it's helpful to have a mental model of how you think the program should work, but you should absolutely count on having to revise that model (and the resulting design) as you learn more. "Plans are worthless, but planning is everything." —President Eisenhower