Even before the pattern. I have a hard time conceptualizing without seeing. I'll grab my knife and literally hold it where and how I want it to ride. I'll literally write down a note on how to do that part, then I keep stepping through the details, figuring out whether I want a belt loop, where to put the loop, do I want a pancake, do I want a dangler?
Since this is your first, I'd recommend a dangler or belt loop vs. a scout or canted rig. You WILL need a welt. I'd make a taco out of the leather with the spine of the knife riding in the closed side of the taco, figure out where the belt loop and welt will go and sew them in. You control the depth. I usually go an inch or so into the sheath or if there is a good amount of curvature on the handle I'll wet form it so it'll retain without a strap.
It seems daunting. I remember my first sheath. You'll learn a ton even though its a little stressful at the outset.