You have two options: download a pre-built release of the compiler, or building it from source.
Downloading a Release
Run this to download and install the latest release:
$ curl -o austral -L https://github.com/austral/austral/releases/latest/download/austral-linux
$ sudo install -m 755 austral /usr/local/bin/austral
Then you can invoke austral from the command line.
Building from Source
First, you need to install opam, the package manager for OCaml. On Debian/Ubuntu, this is simply:
$ sudo apt-get install opam
Then, set up the OCaml compiler environment:
$ opam init
$ opam switch install 4.13.0
Then, download and build Austral:
$ git clone https://github.com/austral/austral.git
$ cd austral
$ ./install-ocaml-deps.sh
$ make
$ sudo make install