I'd suggest a search on Google for 'curtain measuring guide'. Many links will give you the options of different styles of hanging, and the material subsequently needed.
Depending on how full you want your curtains when they are closed, you need at least 1.5 times the width of your window per curtain. If your window is 42 ins wide, that's 2 curtains areas of at least 21 ins - I'd suggest at least 30 ins per curtain finished to give a bit of fullness.
Don't forget to add a couple of inches each side and at the top and bottom for folding over/hemming.
depends on what kind of rail you are using
what fabric you want eg Voile or more substantial material
Are you going to use a curtain tape (ie Rufflrt )
Hmmmm. Which will be probs half my window width then?
Its a cotton material not voile xx
I do want it to look full whilst sitting, and im using a normal
Pole which iv measures using however ribbon ties to attach to pole
So can adjust to suit xxx