In a Linux environment, run ./build_cross_compiler.sh. This will download the code for Binutils and GCC, configure them to target the i686 architecture with no OS, then compile them. This is the compiler toolchain that will be used to compile the kernel.
Next, add export PATH="$HOME/opt/cross/bin:$PATH" to your .bashrc. This will add the newly build binaries to your path so that they can be used from the terminal.
To compile the kernel, run make in the root of the project.
QEMU can be installed using:
sudo apt-get install qemu-system
To run the kernel, use:
qemu-system-x86_64 -kernel kernel.o
If you have trouble running QEMU in WSL, the Windows version can be downloaded here.