Logging into Prompto is pretty straight forward.
You use the same credentials as the Portal when Prompto asks for your email and password.
Depending on your computer you might want to set your quality higher or lower. Prompto relies on the graphical card so to work on the highest quality, make sure you have the right kind of graphics card.
One of the most demanding elements in Prompto is the Dynamic lighting. This lighting makes sure that light behaves as in real life. Making scenes more realistic but also heavier to work with.
If you have problems running your scenes, makes sure you disable dynamic lighting in the settings.
Don't worry, you can always change this setting inside of the scene when working.