There are basically two ways to build a stitch-and-glue boat. If you know the dimensions in advance you can pre-cut the bottom panel as well as the side panels. And then stitch it all loosely together, including stem and transom, and then throw in a few cross-gunwale stretcher bars and boom. You have a boat.
But to do that you do need to know all the gnat's ass dimensions before you even start. How do you get those dimensions in the first place? Boat design software works well for some people. But I don't like it. I particularly don't like the $1000 I'd have to spend to get Rhino. Auto-cad costs even more, while cheaper boat design software is so cumbersome to work with I won't use it.
I like to design new boats by working with full-size models. If you make a minimal number of decisions up front--about side panel dimensions--you can put hinged-at-the-corners and splined-in-the-middle adjustable ribs between the side panels. And then fiddle with widths and side angles until you get what you want. That's what I have always done, since the first boat (of my own design). The photo below comes from a rib set I made close to 30 years ago. I still have them and I still use them.
Even so I always felt I had to predetermine the chine shape as a first step. Was it going to be a straight line chine, or would I dish it out in order to reduce rocker? And if so, how much? And what would be the shape the dished out chine? Other than that one gnarly up-front decision, everything else after that could be adjusted and re-adjusted to get what I wanted. The constant visual feedback from working with a full-size mock up is irreplaceable.
Now the Good Part
It eventually occurred to me you don't even have to make that chine-shape decision in advance anymore. If you make two side panels that are 3-4" inches taller than they need to be, you can attach the adjustable ribs to the side panels so they are flush to a chalk line 3-4" up from the chine edges of the side panels. It's also important to attach those adjustable rib formers at regularly spaced intervals. It doesn't matter exactly where you mark the layout lines as long as they are marked at regular, predictable and repeatable intervals from one end of the boat. Perhaps every 24" from the front end of the side panel. Square those layout marks upward from the chine, at a 90 degree right angle, all the way up to the gunwale edge.
With adjustable rib-formers attached at marked layout locations, as described, with the bottom of each former attached 4" above the chine edge, you are now free to forget about rocker profile during the beginning stages of the design process. You can now concentrate entirely on getting width were you want it and side flare the way you like it. Shaping the bottom edge of the chine panels comes later.
Most drift boat building manuals (including my own) suggest building the boat upside down. But this isn't necessary. In fact I worked out the design of my last boat right-side up. Once I finally got the hull shape looking right, I put a sheet of 1" foundation foam on the shop floor, next to the boat, and then rolled the whole assembly over (from right-side-up to upside-down) by myself, so it was then ready for the bottom panel installation. The soft foundation foam cushions the roll-over. The boat doesn't weigh much at that point anyway (still with no bottom panel attached) so rolling it over by yourself isn't hard.
With a helper it's a snap.
Software is fine. But nothing beats looking at a full-size model--positioned right-side up--that can be adjusted repeatedly until you get it right. Then, once you do get the shape right, you could make a marking jig of some kind (while the hull is still right-side-up) that would allow you to approximate a dashed line along the bottom edge, that gives you the rocker profile you want.
Measure and write down the trapezoid dimensions of each rib former, numbering the stations from 1 to N. Then take the side panels off the adjustable formers. Use a long 3/4 x 3/4 inch straight-grained fir stick to mark a smooth line connecting the dots on the dashed rocker-profile line. Cut one side panel. Use that side panel as a template for the other. That way you know both chine profiles are identical. Now you put it all back together one more time and roll it over. Then you lay a long square bottom panel over the upside-down hull and scribe the edges, where the bottom panel meets the chine. Cut the bottom panel to that line.
Stitch and glue it all together, as per standard stitch and glue techniques. Now you have a custom-designed and hand-made hull.
Designed by yourself.
This way the only thing you have to decide in advance is the side panel length. From that point on anything is adjustable. You get to play with a full-size boat model, in a way that allows you to adjust and re-adjust and re-do everything until you get exactly what you want.
Eventually you look at a hull shape and you just can't keep yourself from smiling. That's the point you know you're ready to mix up some glue.