[DrakilorP2P]

If it's a GUI script I may sketch it in Balsamiq Mockups or on paper.

If it's going to be used by other scripts I often write down how it's going to look to the outside world (the "interface"). These notes tend to transform into documentation.

If it's something complicated I find it useful to write down what it's going to do so I don't overlook something.

On a particular GUI script I didn't know enough to plan much, so I just dumped ideas on a dedicated page of my notebook and tried a lot of things. I had to rewrite it twice.