require('process'); just like any other module.
Works in node.js and browsers via the browser.js shim provided with the module.
The goal of this module is not to be a full-fledged alternative to the builtin process module. This module mostly exists to provide the nextTick functionality and little more. We keep this module lean because it will often be included by default by tools like browserify when it detects a module has used the process global.
It also exposes a "browser" member (i.e. process.browser) which is true in this implementation but undefined in node. This can be used in isomorphic code that adjusts it's behavior depending on which environment it's running in.
If you are looking to provide other process methods, I suggest you monkey patch them onto the process global in your app. A list of user created patches is below.
If you are writing a bundler to package modules for client side use, make sure you use the browser field hint in package.json.
See https://gist.github.com/4339901 for details.
The browserify module will properly handle this field when bundling your files.