I found this 7 step article on wikiHow on how to make a mashup with Audacity. I know I have posted a few Audacity mashup making articles before, but everyone learns differently. I thought it was interesting in that it’s almost a recipe approach. Not a lot of details but it will get you started.
This is almost an overview of the process of doing it, and would be much better if it was accompanied by samples, links to acapella and instrumental sites but I’m sure you can find those links here or on Google.