The AWS guide is still a good starting point for this. The main piece I had trouble with was using aws --version once it was installed.
Prerequisite: have Python3 and Pip3 installed
Quick answer:
Adjust $PATH so it has the path to the AWS executable.
Longer answer:
1) Find where it installed AWS
The guide says to use which python3 - but when I installed Python it installed it to my user, not the system.
When you run pip3 install awscli --updgrade --user it should spit out something like this
... awscli in ./Library/Python/3.8/lib/python/site-packages (1.18.18)
The "./" indicated it is a local path, NOT the system path. You can confirm by checking
ls -la /Users/<insert_user_name>/Library/Python/3.8/bin/aws
2) Edit ~./bash_profile and add the path
export PATH=/Users/<insert_user_name>/Library/Python/3.8/bin:$PATH
3) Reload the profile
source .bash_profile
4) Test - aws --version. It should output
aws-cli/1.18.18 Python/3.8.2 Darwin/17.7.0 botocore/1.15.18