Take down the curtains from the rail.
Take down the rail from the window - the kitchen counter top may be a good place to put it.
Remove the old cord - paying attention to the way it's been threaded. Measure the length
Take a sample to a camping and outdoor shop - or a DIY store - or a good ironmongers and buy a length a little longer than you need.
String it all up on the kitchen worktop and test it. Melt the ends of the cut cord with a match to stop it fraying.
Put it all back together again.