Papyrus Designer is an extension of the Papyrus modeler. In order to install it, there are three different methods
1. The easiest way (if Papyrus is not already installed) is to install the Papyrus SW designer RCP available here. However, this version is not up to date (for Eclipse photon)
2. Install Papyrus Software designer via the market place].
3. Open the standard Eclipse installation dialog (Help->Install New Software). Within, add either the release update site of SW designer: https://download.eclipse.org/modeling/mdt/papyrus/components/designer/ or a nightly build , .
A good starting point for Papyrus Designer are example models enriches with comments. These examples are available via File->New->Example, category "Papyrus" examples.