You can use the flat sheet as an extra layer under the duvet you are lying under or simply in place of a fitted sheet with an elasticated edge. If you use it as a fitted sheet, it should be folded in tightly at the corners, just like an envelope.
How do you know what size sheet you need? If you are using it to lie under, you can consider the following when choosing the size:
240x270 - for beds from 80 to 140 cm wide
270x300 - for beds from 160 to 210 cm wide