You write things the same way. Essentially, plugins in a multi-site environment come in three different flavors:
- Single site plugins
- Network plugins
- Must-use plugins
Single site plugins are installed on the network administration page and activated on each individual site.
Network plugins are installed on the network administration page and activated from the same location. As a result, they're active for all sites.
Must-use plugins are installed to the
/mu-plugins directory and activated by default. There is no UI for activating/deactivating must-use plugins.
The only gotchas you'll run into are the differences between
get_site_option() and similar ... they behave differently for network installations and single sites. But being able to advise you on what to use and how depends entirely on what you're trying to accomplish.
So start with that same tutorial. Then, when you have specific questions about network/single-site functionality, ask questions.