Install macOS Sierra in VirtualBox on macOS host

sierra-virtualbox-install.md

https://www.reddit.com/r/hackintosh/comments/4s561a/macos_sierra_16a238m_install_success_and_guide/

Step 1 (Creating a bootable macOS Sierra ISO for VirtualBox):

1. hdiutil attach /Applications/Install\ macOS\ Sierra\ Public\ Beta.app/Contents/SharedSupport/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_app
2. hdiutil create -o /tmp/Sierra.cdr -size 7316m -layout SPUD -fs HFS+J
3. hdiutil attach /tmp/Sierra.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/install_build
4. asr restore -source /Volumes/install_app/BaseSystem.dmg -target /Volumes/install_build -noprompt -noverify -erase
5. rm /Volumes/OS\ X\ Base\ System/System/Installation/Packages
6. cp -rp /Volumes/install_app/Packages /Volumes/OS\ X\ Base\ System/System/Installation/
7. cp -rp /Volumes/install_app/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System/BaseSystem.chunklist
8. cp -rp /Volumes/install_app/BaseSystem.dmg /Volumes/OS\ X\ Base\ System/BaseSystem.dmg
9. hdiutil detach /Volumes/install_app
10. hdiutil detach /Volumes/OS\ X\ Base\ System/
11. hdiutil convert /tmp/Sierra.cdr.dmg -format UDTO -o /tmp/Sierra.iso
12. mv /tmp/Sierra.iso.cdr ~/Desktop/Sierra.iso

Step 2 (Installing in VirtualBox)

This is pretty straightforward.

1. Create a new El Capitan (there is no option for Sierra at the moment) machine. Leave everything at default
2. Boot the virtual machine from the Sierra.iso you created, which you'll find on your desktop. I had to mash F12 while the machine was starting up and then choose 'EFI CD/DVD' in the boot manager. YMMV.
3. The install process should be familiar to you. If not, you probably shouldn't be playing with a beta OS ;)
4. Grab a coffee, the 15 minute estimation the installer gives you is a complete work of fiction. My install took over an hour.
5. Once installed, reboot and complete the new user process. Skip the AppleID login bit, it'll fail on a VirtualBox.
6. Shut down the virtual machine and quit VirtualBox.

If you are trying this with Catalina you can use this script to create the installer media that can be mounted directly on Virtualbox. More information can be found here.