Because of reoccuring questions about dualboot of GNU/Linux and Microsoft Windows, an authoritative general guide is needed as a reference for users asking about dualboot/multiboot.
Cover both BIOS and UEFI boot methods, using GRUB2 as the Bootloader of choice because it is commonly used in popular GNU/Linux distributions.
Set up the partitioning, with examples for both MBR and GPT partition tables. Include primary/logical partition problems with MBR and shrinking Windows partitions if no space is available elsewhere. This should preferably be explained for GPartEd, as that is the most commonly used GUI partitioning tool. Include information about the special partition that may be necessary with GPT.
Explain setting mount points manually. At a minimum set swap and root (/), but preferably with directions for /home and /boot as well. Add mount points for existing NTFS/FAT/other data partitions. Optionally use the Ubuntu installer as an example so users can adapt to others.
Repair/Install the bootloader when not done automatically by the installer or when damaged. Include the steps potentially needed for secure boot.
The guide shouldn't walk throug all edge cases but be helpful for most users. Part 4 for example should be as easy as showing the usage of
grub-mkconfig (and additional work for secure-boot).