Study guide for "On restricted projections to planes in ${\mathbb{R}}^3$"

This article is a study guide for ``On restricted projections to planes in ${\mathbb{R}}^3$" [arXiv:2207.13844] by Gan, Guo, Guth, Harris, Maldague and Wang. We first present the main problems and preliminaries related to restricted projections in ${\mathbb{R}}^3$. Then we introduce the high-low method and decoupling, which are the two central and novel ideas in their proofs. We hope to provide as many details as possible so that this study guide is self-contained, with the only exception of the Bourgain-Demeter decoupling inequality for curves in the appendix.