Since we moved to a new home. I needed to redo my daughter's curtains. The new window is narrower and longer than the last window. So, I needed to lengthen the current curtains. Even though her room is on the north side of the house, a lot of light still penetrates through the curtain. Thus I decided to make it reversible from an already made curtain, causing it to be made from 2 layers of thick fabric. If you would like to completely block out the light, I suggest using blackout fabric.
All I did was sew new fabric on to the back of an already made curtain. First, I pressed the new fabric 1/2" on the two sides and top. Now, for the bottom I pressed under 1/2" of the raw edge then folded again 3" from the bottom and sewed. Why 3"? So it would have the same border bottom as the current curtains.
Now pin the "new fabric top" to the already made curtain. Next, I sewed along the edge of the two sides and top of now joined curtain. This is a really easy way to get curtains you love without having to make a whole new curtain.
Tip: Make sure not to sew closed the rod hole.