To install a package, login as an Administrator that has Package Install permissions and go to System > Package Manager. There is a form there where you can enter the package name of the package you wish to install. Package names must be all lowercase, the names can be found at the top of each package description below. Click on any of the packages to get a full description with setup instructions.
Any Laravel (a PHP framework) developer can build an add-on package.
Please consult the documentation for instructions on building a SynergyCP add-on.