Do you mean you want to create a partition for it or you want to overwrite an existing partition? If a new one, the general idea is you shrink Windows 10 first (from Windows!), then you begin the install and allow it to use the largest free space or some such option. It will then automatically partition. If you are replacing one, all you have to do is use the installer to make that the / partition. These are the simple instructions. If you want more detail, you'll have to explain more what you are doing.
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