My foundation was going to consist of basically two parts. The bottom layer of block was called the Footer and was 16 inches wide. Then on top of that lay the actual foundation walls. The block for the foundation walls were turned so they were 8 inches wide instead of 16. So basically the narrow foundation walls sit on top of the wide footer.
The footer is the most critical piece of the foundation, because if it isn't square, flat, and level, you will be correcting for those problems throughout construction. And believe me, you don't want to have to be applying corrections to all your measurements, it's complicated enough when everything is perfect.
The way I handled it was I bought a few truck loads of block to begin playing with. I got my block from a place called Sutherlands, where I could drive my truck up to a full pallet of blocks (which is 75 blocks) and they would load the pallet into my truck with a fork lift. 75 blocks is very heavy by the way, make sure your truck can handle it.
I began laying a test footer just to see how things were turning out. The trick to making the footer square is to measure the diagonal corners. They should measure EXACTLY the same. If they are off, your garage is going to be a trapazoid or something, and you don't want that.
It was at this point when I first noticed that my blocks were not lining up to be a perfect 32x24. But then I realized that I was going to be putting foundation walls on top of this footer and they were not going to be 32x24 either. I had to lay out in my head (and on scratch paper) how many blocks would be needed for each leg of both the footer and foundation walls in order to get the garage walls as close to (but not over) 32x24 as possible. Also, it's important to know that the wall framing rests on the outside edge of the foundation walls. This is to prevent water from sitting on the foundation and rotting the framing. So it is the outside edges of the foundation walls that should come as close as possible to 32x24.
Once I had that figured out, I knew I had to fire up the excavator again and adjust my trench slightly based on my new calculations.
I laid out my test run of footer blocks, and they were terribly crooked and nowhere close to level. But that was okay, that's why I was testing. I made some notes of areas I needed to dig slightly differently, re-estimated everything to within an inch of exact, pulled up the test blocks, and fixed my trench so it was as close to perfect as an amateur could get it.