You take something that is at least 40cm long and straight and saw along or draw a line along it. Then use the angle square to get the other sides perpendicular to the first one.
Does that sound like a way forward?
It might be a good idea to cross-measure diagonally before you saw. The distance between the corners should be the square root of (forty squared plus forty squared).
Does that sound like a way forward?
It might be a good idea to cross-measure diagonally before you saw. The distance between the corners should be the square root of (forty squared plus forty squared).
you should find a right angle and start from there. A sheet of paper for the printer usually has right angles... so you can take one if the size is suitable... then draw all corners at right angles... that's how I would have done it, in some way 
gaia
gaia
What you need to do is create a straight square if I understand you correctly.
The trick then is to "forget" the outer edges.
So, draw a straight line using a ruler, for example. It doesn't matter at all how this line relates to the outer edge since you've forgotten this (right?).
Then, as mentioned, take the angle square and measure two new lines from your first line.
Now you have three sides, and it's just a matter of measuring in the fourth side.
The trick then is to "forget" the outer edges.
So, draw a straight line using a ruler, for example. It doesn't matter at all how this line relates to the outer edge since you've forgotten this (right?).
Then, as mentioned, take the angle square and measure two new lines from your first line.
Now you have three sides, and it's just a matter of measuring in the fourth side.