Link
Here is the Guide for it. Make sure you have everything backed up and a recovery disk handy. Partitioning a HDD can occasionally go wrong.
"Manual partitioning
Choose "Manually edit partition table".
Listed will be your current partitions.
Select the partition you want to resize and press Enter.
Select "Size:", press Enter.
Select Yes, press Enter.
Type in a new size in gigabytes for your partition, it's recommended you free up at least 10 GB of free space for your Ubuntu install. Press Enter when happy with your changes. It may take some time to apply the changes.
Create a swap partition of at least your amount of RAM (if you don't know, 8000 MB is a good value).
Create a partition for your Ubuntu installation.
Create other partitions if necessary: see DiskSpace
Select "Finish partitioning and write changes to disk". "
Let us know if you get stuck on any particular step. I have installed Ubuntu half a hundred times and would be more than happy to provide specifics