To begin with you need to get npm installed properly. DO NOT install this from Ubuntu official repository. It will give you all sorts of trouble later on upgrading and stuff.
I do not remember how I installed the stuff, but the ideal installation will let you update npm packages without sudo. This will involve placing NODE_PATH in ~/.bashrc and I am sure you can find out how to do that on your own.

But once you have node and npm correctly installed, installing docpad is a piece of cake. npm install -g docpad

FOr my workflow, you need yeoman generator also. npm install -g yeoman. It is then invoked via yo command

Workflow

The workflow I use is the following.

One time steps

mkdir my-project
cd my-project
yo docpad

During the yo interactive choices, I choose gh-pages plugin also. This helps me deploy the site to github-pages easily. The gh-pages plugin might need a small configuration (which branch to push to). This is gh-pages by default. But if you are hosting an organization repo, you will need to change that to master (after renaming the source branch to "source" maybe?). Check my docpad.coffee file.

You will have to adjust various other settings in docpad.coffee and also change the layout and other things.

Additionally, I create an alias in ~/.zshrc (or ~/.bashrc for bash people),alias cdgh='git push && docpad deploy-ghpages --env static'. This is useful in pushing all changes to source branch and master (or gh-pages) simultaneously.

Now we are ready to hack away.

Each time steps

Every new post I need to make, I do just this.

vim src/newfile.html.md  #and edit that
git add src/newfile.html.md
git commit -m "added a newfile"
cdgh #our earlier alias