Papyrus Designer is an extension of the Papyrus modeler. In order to install it, there are three different methods (CAVEAT: currently only the nightly update site are working properly with a Papyrus 2020-12 or newer, for 2020-06 the associated nightly update site is recommended as it contains several bug fixes and enhancements that are not yet in the release)
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 model containing comments. These examples are available via File->New->Example, category "Papyrus" examples.