If you just want to play Polished Crystal, use one of the .ips patches as described in README.md. To build a ROM yourself from the source code, follow these instructions.

You know it's sitting back and looking at it that you realize that Gamefreak is actually really good devs.
Like you have Pokemon and it's a simple formula "How can you fuck it up?" and then all of these Romhacks basically go "Like this."

— Anonymous, /vp/

To build on Windows, install Cygwin with the default settings.

In the installer, select the following packages: git, python, make, and gcc-core.

In the Cygwin terminal:

git clone https://github.com/Rangi42/polishedcrystal.git
cd polishedcrystal
mkdir rgbds

Then download rgbds. If you're building Polished Crystal 2.2.0, get rgbds version 0.3.9. If you're building the 3.0.0 beta, you'll need version 0.5.2. Extract the archive and put all the .exe and .dll files in polishedcrystal/rgbds.

To build polishedcrystal.gbc:

make

To build other versions:

make [faithful] [nortc] [hgss|monochrome|noir] [debug]

For example, to build a mononchrome faithful version:

make faithful monochrome

Python 2.7 is required.

sudo apt-get install make python gcc git bison

git clone https://github.com/gbdev/rgbds.git
cd rgbds
sudo make install
cd ..

git clone https://github.com/Rangi42/polishedcrystal.git
cd polishedcrystal

To build polishedcrystal.gbc:

make

To build other versions:

make [faithful] [nortc] [hgss|monochrome|noir] [debug]

md5sum, ghead and gtail are required.
To install it: brew install coreutils

In Terminal, run:

xcode-select --install

brew install rgbds

git clone https://github.com/Rangi42/polishedcrystal.git
cd polishedcrystal

To build polishedcrystal.gbc:

make

To build other versions:

make [faithful] [nortc] [hgss|monochrome|noir] [debug]