Bella Engen recently posted a useful 101 walk through on how to use a custom Thesaurus for SharePoint 2013 Search which you can find here.

Adding a thesaurus can be incredibly useful to get the most out of SharePoint Search, because not all users search for content with the same keywords or phrases.  Furthermore, as your business evolves, you will typically have product, service or business unit names change over time – and having thesaurus can help unlock this… however it is important to know that you’ll often need a combination of a Thesaurus + Query rules, which Steve Mann explains in detail here.