You'll find many of the tools you need to set up your environment at https://flutter.dev. Execute the following steps:

1. Click on a very visible Get Started link or button on the page.
2. From there, you'll have to choose your operating system (in this case, macOS), and then download the Flutter SDK, which is a .zip file. Create a folder for the content of the file, such as ~/flutterdev.
3. In order to create a new directory on the root of your Mac, open a Terminal window, and type the following command:

mkdir ~/flutterdev

4. Next, extract the content of the zipped file into the new directory. From the Terminal, type the following code:

cd ~/ flutterdev

$ unzip ~/Downloads/flutter_macos_v1.2.1-stable.zip

5. You may need to change the Flutter SDK version, depending on the version available at the time you perform the installation.
6. At the end of the process, you should have a folder containing the flutterdev folder, with its files.
7. Next, you'll need to update your PATH variable. This will allow you to run Flutter commands in any Terminal session.
8. From a Terminal window, go to your root folder by typing cd ~/.
9. Then, if you haven't created a .bash_profile yet, type the following command:

touch .bash_profile

10. Create your new file. Edit .bash_profile with your favorite editor, or type the following command:

open -e .bash_profile

11. In the file, type export PATH=/home/flutterdev/bin:$PATH and then, from the Terminal, run source $HOME/.bash_profile.
12. Check whether the PATH has been correctly updated by typing $ echo $PATH.

You will see the path you've set on the Terminal.