I´m working on many WordPress projects for myself, and for many different clients. This could be a nightmare on organization going from site to site with all of the css, themes, pluggins, activated widgets, php, log ins etc.
With WordPress, you can plug in pretty much 10 lines of code into your WordPress wp-config file and make it so that you can work on a thousand separate WordPress sites all at the same time on your public server, or better yet on your own local hard drive.
Though the process is really simple if you know what you are doing, one wrong step can really make for a very long night.
I found this very simple guide. Follow it slowly, and step by step.
I use Adobe Dreamweaver CC to edit my WordPress themes. Once the WordPress Multisite is ready to roll, you can define each site separately in Dreamweaver and edit the css site by site and it´s lot of fun.
Here´s the guide for your WordPress Multisite setup on Mamp, Enjoy.