To dual boot Pop!_OS alongside another OS install Pop!_OS first. Then once booted into Pop!_OS use GParted to resize the root partition (the largest partition) to make room for Windows 10 (35GB is the minimum required).
If you wish to install Pop!_OS here is a link to it.
Once Windows 10 is loaded and the ‘Windows Setup’ window is shown the partition that was created earlier for Windows can be selected. When installing Windows 10 it will use the Pop EFI system partition (ESP) so that Windows boot efi files will be placed in the ESP partition.
With the ‘Unallocated Space’ selected new press the ‘Apply’ button. Note if the whole partition is being used for this installation then the size property won’t need to be changed.
Next Windows will create an extra partition that is needs for some features.
This message can be safely ignored and it is caused by Windows not being the first OS in the boot order.
Another way to set up a dual boot is to install another drive for the other OS of your choice. This is one of the easiest ways to dual boot as each OS will set up the whole drive for automatically created partitions and won’t require you to resize any partitions. To access each OS you would reboot and hold the boot menu key (F7 for our laptops and F10/F12/Del for our desktops).
NOTE: If you feel comfortable opening your machine, it may be helpful to remove whichever drive is not undergoing changes while you install the OS on the opposite drive. For example, removing “drive 1,” while installing Windows on “drive 2.”
This will prevent Windows (or Pop!_OS) from changing the boot partitions of the opposite drive. This is an extra precaution and not usually necessary; however, Windows does not always “play well with others,” and removing drives you do not want changed during installation insures against this.
Windows 8 and later uses a “Fast Startup” setting which prevents Windows from fully shutting down and allowing other OSes to use the disk. Before you can properly dual boot with Windows, you must disable this setting in Windows.
In your Windows install, open Control Panel and head to “Power Options” Select “Choose what the power buttons do”, select “Change settings that are currently unavailable”, then disable the “fast startup” setting. Note that Windows updates may occasionally turn this setting back on without asking, so if you are unable to boot into Pop!_OS, check this setting first.
Once in the BIOS change the Boot Order to boot Pop!_OS first that way when the Spacebar is pressed systemd-boot will appear and then the Windows Boot Manager can be selected for booting Windows 10. Steps for accessing the BIOS and changing the Boot Order are found here.