If this is your first time building it, you will need to first open a terminal in the folder with the source code and run rustup override set nightly && rustup update nightly --force. This will tell Rust to use the nightly version, since this uses some features only available in the nightly channel.
To make an image you will also need to have bootimage installed.
Use this command to install it:
cargo install bootimage
You will also need the LLVM tools. Install using:
rustup component add llvm-tools-preview
Also install QEMU to be able to run it in a virtual machine.
To build the OS and make a bootable image use the command:
cargo bootimage
The image will be located at target/x86_64-flow_os/debug/bootimage-flow_os.bin
Use QEMU to boot it up with the following command:
qemu-system-x86_64 -drive format=raw,file=target/x86_64-flow_os/debug/bootimage-flow_os.bin
Additionally, you can use cargo run to build and boot Flow in QEMU