Basically, follow the installation guide in the node.js GitHub wiki page. I use the "Building from source" guide. Then install npm, follow the guide too, just easy type with "curl http://npmjs.org/install.sh | sh"
When I test nmp installation (install express), I found that I should type 'npm install -g express', and the express will be installed in the directory which I want. Otherwise, nmp will install locally (such as: ./node_modules/).