https://help.ubuntu.com/community/HowtoPartition/CreatingPartitions

Here is a good manual for creating partitions using Gparted, and includes pictures.

First, install gparted from the software center, or via command line:

$ sudo apt-get install gparted

Run gparted from your applications, you'll be asked for your root password.

You'll be presented with a list of your exsting partitions. You can re-format a desired partition by right-click and unmount, then right click, and choose a new filesystem. Once done that - click on the green tick above, that will write changes.

You should then have a valid ntfs partition to install Windows. Good luck!