Hi,
Using the Something else option will allow you to select the partition you wish to use for installation.
Was about to write a detailed step-by-step complete with screen captures, then found wolfman’s great guide (linked above) 
If you have more than one drive, make sure the correct drive is selected for the boot loader (found at the bottom of the Something else window).
Once you’ve completed the first part of the guide (formatting your preconfigured partition as ext4, Mount point: /), if you do not wish to have a separate /home partition, you can continue with the installation as normal.
During install, GRUB will automatically be updated to include all operating systems found on your system.
As with all partition operations, make sure to backup important data before commencing.