Given that UiPath's platform is built on .NET, all activities take the form of a Nuget package (this is the .nupkg file you get when downloading an activity directly from UiPath Connect! or from the feeds in UiPath Studio's package manager). Let's walk through the steps to build a .nupkg file and import it into UiPath Studio for use.
Within the Solution Explorer, right-click your Design project and select Publish. This option builds your projects, packages them together, and sends the package wherever you'd like.
In the next screen, enter the folder in which you'd like to save your activity package. It's a good idea to choose one of your package sources from UiPath Studio (you can find these at Settings > Manage Sources on the Studio homepage) so you can publish directly from Visual Studio into UiPath Studio.
Click Create Profile to continue.
On the next page, rename your package source to UiPath Packages (or whatever you'd like) and change the configuration to Debug. Then click Publish.
For more info on Debug vs Release configurations, see Package Metadata.
Your output will then show that all three projects have been built successfully and the resultant package published. You now have a working activity package!
Open UiPath Studio, navigate to the Package Manager, and add your activity set to workflow.
Notice that a new category has been added to the Activities pane.
That's all! Give your new activity a try!