Kudos to the forum for not immediately recommending you contact XYZ seat builder, "they did a great job and blah blah"... this is a builder's forum after all.
Good on you for taking this on yourself. Like any new skill, it will take some time and in this case, some wasted material (think of it as the price of education). The previous ideas about using some practice material (something of the same weight and weave you intend to use would be ideal), asking for advice at a supplier like JoAnn's, and watching youtube videos for the basics. You'll also want to look into renting/borrowing or buying the right machine. A simple single needle walking foot is what you want. From there the options go to the moon.
Regarding those pesky corners, there are a bunch of options depending on the final "look" you're looking for. The simplest is called a "boxed corner", you can modify that slightly to make a rounded box to contour to your foam. An angled ear corner might work as well. You'll figure it out.
There are a ton of techniques that will make your task easier: consistent hem margins, good pinning or clipping, thread size to match the fabric, foot pressure etc...
The seats in my -7 are Sunmate foam from Dynamic Systems Inc., and the material structure is dependent on a good sub base shape. Sounds like you have your foam already. My covers are just like you'll see in ANY Boeing plane, wool twill around the sides and Aussie sheep skin for the tops. They rock. Be happy you're sewing fabric and not sheep hide!
Some of the accessory pouches and stick boots are leather, another branch that has it's own peculiar requirements.
Keep at it. My guess is that you can buy a good machine and do this yourself for the price of having someone do it for you. When you're done you own a new skill that can never be taken and a sewing machine for the next project! Build on