Pyrrha knows that this is important to Weiss, so she decides to propose before Weiss can propose to her. She knows it would mean the world to her.
She wants to make it special for Weiss, wants to show her how much she loves her. She gets Blake and Yang to help her, not Nora because it would be so hard for her to keep quiet about it. Yang takes Weiss for a shopping trip while Blake and Pyrrha set up candles and rose petals on the roof top.
Roses and candles are cheesy, romantic and what Weiss would want. The rooftop and stars are what reminds Pyrrha of their relationship, what makes this special for her. She has a silver ring with a red gemstone, goes down on one knee and Weiss cries when she says yes.