Once the partition is created and assigned a drive letter, the next step is to install the OS. During the install of the OS you will be given the option to choose where you want the install to be located. The key to that is to choose the custom install path. Youtube has plenty of video's on the subject.
To be clear, It's a VERY bad idea to dual boot Windows 7 with 8.1.1… Windows 7 is a legacy system and they just aren’t that compatible.
If you have a laptop and need windows 7 then hyper-v allows you to run it virtually without graphics.
If you have a desk top then a multi-boot set up allows Windows 7 and Windows 8.1.1 to each have the file structure designed for them, each hdd has it's own swap file and you need to keep windows 7 away from the Windows 8.1.1 C drive… I still don't recommend it unless you are very clear on what system is allowed to access which files.
I've been running this setup since 8 came out on my laptop....I've never had and issue. Windows 7 Home Premium is the primary OS (OEM)....once I partitioned the HDD and clean installed 8.1....the 8.1 boot order menu appears at startup rather than the windows 7 black generic screen for dual booting and I choose my OS. Works flawlessly for me.
I assume you have it in legacy mode... press [windows key] + [r] and type "msinfo32" (without the quotes) to confirm. If so then 8.1 isn't working at its full potential is all I'm warning about. It’s your system guys and more power to you but there are pit falls; slow boot times, old file formats and less security is inherent to legacy.