I enjoy reading "process posts" but it has never occurred to me to write one of my own. This isn't going to be that post - but it is the start of one!
Ideas usually come to me fully formed, in other words I 'see' the finished piece then work out how to create it. This doesn't mean that the start and finish are identical - I take liberties with the idea as I go, but this is how I begin.
I pull the colours from from my 'stash' or use an exisiting fabric and re-colour it with dye or paint to get what I want. During the same period I use Coreldraw (I'm on version 9 which we've had for an eternity but it does everthing I want!) to work out dimensions. I can see the proportions in my head but have no idea what size things actually are. I draw the shapes then use the measuring tool to check actual dimensions. There's a lot of trial and error and/or auditioning that goes on at this stage - there's little point fixing on something if I've to cut a piece of fabric 9.275" x 8.32" because it will never happen!
In this example, from my January TIF, I've tried a different orientation and used the colour palette just to check the contrast of the (approximate) intended colours.
Sometimes I take this further and mark in potential quilting lines but usually I wait until the construction is done to make those kind of decisions. If you enlarge this image you'll see that I had considered adding some lettering, but once I had the JQ under the machine it became clear that this wasn't an option. But I'm sure that it will happen at some point in the not too distant future!