Go to this link and follow the instructions. https://getcomposer.org/doc/00-intro.md#installation-windows (this is not a guide to install composer :))
C:\Users\username>composer -V
Composer version xxxxxxxxxx
(this is not a guide to install PHP :))
Open the terminal and download its installer:
curl -O https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar
Check we have it in the current folder:
php wp-cli.phar --info
C:\Users\username>echo @php "%~wp-cli.phar" %*
Go to System’s Properties -> Advanced System Properties -> select the tab labeled Advanced. -> Click Environment Variables, under the System variables section scroll down to the variable named Path and add ;C:\wp-cli
C:\Users\username> wp