A few careful measurements help you explain exactly what you need. Use the same unit throughout.
01 - Measure the width
Record the width of the rod or track, or the area you want the curtains to cover. State whether this is the total area or the finished width of each panel.
02 - Measure the drop
Measure from the intended hanging point to the desired finish. The starting point depends on the heading style.
03 - Confirm the details
Include your unit (cm or inches), number of panels, preferred heading, lining and fullness. Have the seller confirm the finished dimensions before ordering.