The trick for learning how to build a storage shed is to stretch the line across a scrap of wood at each end so there’s a space between the line and the part you’re straightening. On a wall, for example, tack small scraps of 1/2-in. plywood at each end of the top plate and stretch a line very tightly over the plywood. Then use a third scrap as a gauge to check the distance between the line and the lumber. Use braces or whatever is needed to adjust the top plate until the gauge just fits. Now your top plate will be perfectly straight.
Door placement is also important. You often see doors placed on the gable end of the building, which looks nice, but makes it virtually impossible to reach items stored at the rear of the shed. A better alternative is to put the door on the long side wall, so that you'll be able to access items to the right, left and back. Another option is to install doors on both gable-end walls, so that you'll be able to easily reach items from either end of the shed.
3 The frame of the roof consists of four 2-by-4 rafters cut so that they rest on the front- and rear-wall top plates, as shown in the rafter detail at right. Install a gable stud directly over the center studs on both side walls to fit between the end rafter and the top plate. Install the sub-fascia and then the fascia to the front ends of the rafters.
I got suckered into the 12k shed plans, only to find myself falling for one of the oldest scammer sales pitches in the book by being up-sold into the add-ons. Before I clicked I did a quick review and the first few sites that popped up had said that it was worth the money to buy the add-on because that is where the real value was (perhaps a review page planted by the good folks of MyShedPlans).
I don't waste my time on free plans, store-bought sheds, or ready-made, mail order kit solutions... been there, done that. Free means lack of user feedback and you get absolutely no customer support. With store-bought and ready-made mail order sheds: they're flimsy, profit-driven solutions that I've always had to tear apart and modify too much to get the storage space to work or to match the character and style of the home.