I’ve just written a tutorial over at Macs in Chemistry on how to create a Safari Extension. It’s aimed at beginners, with no prior knowledge required. A bit of experience with JavaScript will be useful to really understand what you are doing, but this is something you can pick up along the way.

The tutorial guides you through making an extension that searches the PubChem database, but you can easily adapt it to work with any search engine you want. If there is interest I can write a follow up that addresses some more advanced concepts like automatic updating and injecting content into web pages.

If you are interested in Safari Extensions check out the Mac OS X Tips Safari Extensions collection, and feel free to copy any of my code.