All custom size items have the following sales terms:
One of the easiest ways to draw a diagram for an irregular shape is to use grid (graph) paper. Since the manufacturer is in Germany and uses metric measurements everywhere, it's simplest to use a 1cm grid paper and a scale of 1:10 (one cm grid = 10 cm in real life). If you don't have some on hand, Google search 1cm grid paper and print a sheet. If you're more familiar with the standard/Imperial measurement system (or that's the only tape measure you have), Google convert in to cm and convert as necessary.
Once you've drawn out your entire shape, make sure either every side is measured or there are angles marked clearly where necessary. In the example below, the entire irregular shape fits inside the marked rectangle 150cm x 70cm. You can use the calculator for a standard rectangle of those dimensions to get a price quote. If you're satisfied, scan or take a photo of the complete diagram and send it to firstname.lastname@example.org. If you require further assistance, feel free to contact us, too.