I am in the initial planning phases of building an attached garage.
Has anyone created their own foundation for such a structure.

We built our own detached garage a few years ago. My wife and I poured
the footing and perimeter wall ourselves, then hired out the slab work
inside.

Our site was mostly level, so I formed the footings using 2x8's and
2x10's (I later ripped these into 2x6's for studs and blocking. Nothing
went to waste). We were required to install a row of rebar in the
footing, as well as vertical rebar stubs every 4 feet to tie in the wall.

After the footings were poured, I made wall forms using 3/4" plywood and
a 2x4 framework. Each panel was 2'x8'. I fastened these to the existing
footing forms, and tied the tops together. I used long bolts every 4' or
so to tie the inner and outer forms together, through a short piece of
pvc pipe inside to maintain the correct thickness. It worked great. We
also had to install another row of rebar and the anchor bolts at the top
of the wall.

Pouring the footing and wall was frantic and intense work. But it only
took about 30-45 minutes for each pour. We stripped the forms 7 days
later.

For the slab, we set up the forms, installed a 6-mil vapor barrier, and
compacted a gravel base ourselves. The professional crew simply poured
and finished the slab. They had four "experienced" guys working their
butts off as fast as they could. There's no way my wife and I could have
pulled that off on our own. We used fibermesh additive in the 24'x28'
slab, and they did an excellent job of finishing the slab. About five
years later now, and no signs of any cracking.

We reused the wall forms to build the foundation for our house, then I
sold them to a farmer who was going to build shelters for his animals or
something.

As for grading the land, I rented a small bobcat to level the site where
we built our house. It was mostly level anyway, just needed a foot or so
of fine tuning here and there.

I used a simple water level to get the site and foundation forms level.
Worked great.

Good luck!

Anthony