I always start my projects with a sketch so I can think everything out thoroughly.
See? Super easy. Just make a wide rectangle the width of your window, it doesn't have to be quite as long because it will be tied up. Seam up the two sides and the bottom and make a tube at the top for the rod to go through.
For the ties, you can either use ribbon or make some out of the fabric you are using. To do this, just cut long strips twice as wide as the desired ribbon, fold in half and sew right sides together and turn it inside out.
Sew the ribbon half-way down your curtain on both sides (front and back). You may want to pin it down to get the spacing even and to make sure it doesn't move at all while you are sewing.
Remember to leave the ends of the ribbon long for tying bows when you are done!
And you're done. Easy, right? If you are unsure about how much fabric to buy, just measure your window and take the measurements with you to the store. Let me know if you make one!