With a foundation on level ground squaring of a section is pretty easy to do and can be done without having sophisticated surveying equipment onsite. Placing footings on a slope becomes a bit more involved but if the slope is within reason and with the use of a step ladder a straight piece of lumber and a level it will work the same way as you do it on level ground.
I should add, getting a surveyor to come out and pin all the corners of the foundation would a be a good plan. Especially if you are not used to doing this kind of work. Get him to place a marker (a large nail with a ribbon) on corner locations of the foundation wall. Get him to mark as many corners of the foundation as you require. Off sett the outside footing form boards from these pins and away you go. This will save a lot of time and worries building the foundation. No need to find the property pins, which can be an adventure on its own. No need having to string a line between property pins to measure the setbacks from. And no need to read any further on how to make sure that your footings are placed in the right spot and square.
Zoning by-laws are a set of regulations governing what the land can be used for and one of the things they cover is the setbacks required from property lines to the foundation of your home.
In certain cases where the second floor has a cantilever, the required setback may have to be measured from the property line to the outside of the upper floor. Make sure you check this!
The required setbacks should be on your building plan. When you have this information you are ready to start.
At the very least you will have to locate your two front property pins and at least one of the rear property corner pins.
It is always better to locate all the property pins and measure from pin to pin to make sure that they are in the right spot as per your plot plan.
Next, you need to string a line between the back and front pins to be able to measure your setbacks. It is not always necessary to string a line between the front property pins but if this property line does not run parallel with the building or you have something projecting from the front of the building you may have to do this as well. Keep in mind that the required setback is from property line to outside foundation wall, not the footings.
Starting with corner A, find the required setbacks for the front and the side. If side set back is 6' to the foundation wall then for 18" footings (inside measurement) and an 8" wall it would be 5' 7" to the inside of the outside board of the footings. If the front required setback is 20' then it would be 19 ' 7" to the inside of the outside board of the footings. Stake this corner in place so it can't move anymore. Do the same for corner B. Get the offset needed for the side and stake it. Run a string line along the inside of the outside board from A to B slightly above the board, and after straightening it out stake this side. You now have one straight section in the right place. Construct the rest of the footings if you haven't done so already, kind of making sure that they are close to being in the right spot. Next step before squaring this section is to check to see that the lengths A - B and D - C are identical. Check the same for lengths B-C and D- A. If they are not, fix them so they are. No use squaring things if one or both are of different lengths. Now measure diagonally from the outside of corner A to corner C. Remember this measurement. Do the same for corner B to D. If the length A-C is longer than B-D, say by 2", split the difference and move wall C D down one inch. Keep repeating this till both the A C and B D diagonal lengths are identical. Kind of make sure that when moving things up and down that the footings sections affected by the move stay reasonably straight. Accuracy counts here. You do have a bit of room for error, but much better to start of square from the start!
After you are happy with the measurements stake all the corners and with a string from end to end straighten the sections out placing more stakes along the sides as needed. If you have done a perfect job with the excavation it should be easy to level for height, nail the stakes as you go along leveling the footings with a laser or builders level.
This is based on that 3 squared plus 4 squared = 25 take the square root of that = 5. Squaring this way works and most people will be able to remember the required measurements.
At times it will be more efficient and more accurate to square an L shape using the total available lengths. Not as easy as using 3 4 5 as the measurements and it will take a calculation to get the diagonal length.