You can install mocha either globally or in your project folder. The latter is the preferred way.
In all the example let's assume that all the test files are in a test folder within the project folder.
To install mocha in your project folder, you can use the following npm command:
$ cd my-project/folder
$ npm install mocha --save-dev
This command will install mocha inside the node_modules folder in your project and add a (development) dependency entry inside the package.json file.
To use mocha from the CLI you can either use the mocha command inside the ./node_modules/.bin/ folder:
$ ./node_modules/.bin/mocha ./test
Or use a npm script (a npm script uses by default the commands in the .bin folder).
# package.json
{
 "name": "my-project",
 "version": "0.0.1",
 "description": "my first tested project",
 "scripts": {
    "start": "node app.js",
    "test": "mocha ./test"
 },
 ...
}
To call that script you can do now:
$ npm run test
Or simply (test is a special script in npm):
$ npm test
To use mocha inside a webpage (just front-end), just include the mocha.js file inside node_modules/mocha/mocha.js inside your webpage:
## HTML page with tests
<script src="node_modules/mocha/mocha.js"></script>
For a global install use npm as follow:
$ npm install mocha -g
This will install mocha in your global environment and bind the mocha command to your CLI., so you can call mocha from the terminal in any place.
$ mocha ./test