Incapsulating Plugin

Hallo i would like to make a new free theme , including a free custom plugin for the portfolio. The problem , simple, is that i would like to integrate the plugin inside the theme (Simple Portfolio inside twentytwelve) , not as separate plugin to be installed. I ‘m looking for a tutorial , some sugestion? ( the wordpress one is too much detailed, i was looking for something more focuzed) . Thanks a lot!