Let's start with our volume interfacing. I like to use a felt interfacing as it gives the bag great structure. I use CK0200 Felt White from Ceka Textilvertrieb (they have English speaking staff to help you). This will become our pattern template for the other bag pieces too.
Cut a rectangle 55cm wide x 30cm high
You need to work VERY precisely, or your boxy bag will resemble the leaning tower, and you will not be happy!
Mark the centre line of the 55cm width with a pin either end. Fold each 30cm side in towards the centre markers, and pin them in place. You now have a rectangle that looks like this
Use your ruler to draw 5cm x 5cm blocks in each corner
CAREFULLY cut these corners away
And, voilà, you have your pattern!
Now you know how to make this simple pattern, you can increase or decrease it's size. Keep the corners you cut out proportional to the size you make, adding or subtracting roughly 5mm for each 5cm you add to the 55cm end, and 5mm for every 10cm you add / subtract to 30cm end
Keep them SQUARE!!!
Bear in mind: if you are using a fabric with a pattern that has a clear 'right way up', then you will need to divide this pattern along the centre line (that you first marked with pins), and cut two pieces of this, adding 1cm for a seam allowance.
Make certain you use 1cm seam allowance throughout
Make this easy for yourself by marking 1cm from your machine's needle with a piece of black electrical tape, giving you a nice, clear line to follow.