Wish you would have posted this a week ago....had last week off.
You start with goals to be met: output/extension/max cab size.
Then you pick a driver, and if you trust the published T/S parameters, you go into hornresp and start simulating horns. Trial and error will lead you to a design you can be comfortable with. Then you need to fold the horn up, a simple, unfinifhed tutorial (aren't they all?) is here:http://www.avsforum.com/avs-vb/showthread.php?t=1222472
NOTE: I use PAR instead of CON for horn profile/flare, as PAR is the way I build, with parallel sides and non-curved tapers.
It takes a while to learn how to play with hornresp and Sketchup, but you can design a decent horn for yourself....you must realize the limitations of horns. They get output, but they trade bandwidth, irregular freq response and size in return.