First you need an ISO file of the macOS installation DMG. Either download that file from the internet (please be advised to use safe sources) or you follow a tutorial to convert the DMG to an ISO. A tutorial is given here:

https://www.xda-developers.com/how-install-macos-virtualbox

Simply install macOS by adding a virtual machine an selecting the ISO and system as macOS. After the VM was added, change the config as follows:

• Min 2-coreCPU
• Min 4 GB RAM
• Min 40 GB HardDisk
• Min 128 MB VboxSVGA with 3D acceleration enabled

Last steps, you need to execute special config commands in terminal:

$ VBoxManage modifyvm "VM_NAME" --cpuidset 00000001 000106e5 00100800 0098e3fd bfebfbff

$ VBoxManage setextradata "VM_NAME" "VBoxInternal/Devices/efi/0/Config/DmiSystemProduct" "iMac11,3"

$ VBoxManage setextradata "VM_NAME" "VBoxInternal/Devices/efi/0/Config/DmiSystemVersion" "1.0"

$ VBoxManage setextradata "VM_NAME" "VBoxInternal/Devices/efi/0/Config/DmiBoardProduct" "Iloveapple"

$ VBoxManage setextradata "VM_NAME" "VBoxInternal/Devices/smc/0/Config/DeviceKey" "ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc"

$ VBoxManage setextradata "VM_NAME" "VBoxInternal/Devices/smc/0/Config/GetKeyFromRealSMC" 1