You need to have some prerequisits:
apt install build-essential gawk unzip libncurses-dev libz-dev libssl-dev wget git subversion
Check out this repository and execute make, i.e. like this:
git clone https://github.com/ffrgb/site-ffrgb.git site-ffrgb
cd site-ffrgb
make
Some common useful parameters for make:
GLUON_TARGETS=ar71xx-generic - Only build the specified architecture
GLUON_GIT_REF=v2018.2.x - wich Gluon-Release version to build
FORCE_UNSAFE_CONFIGURE=1 - If You build as root in an disposable environment You don't care about
BROKEN=1 - Also build for untested but defined hardware wich may not work
V=s - Print more information
Think about starting Your make process inside a screen session as it usually takes a long time to build.
Look at the site configuration related Gluon documentation for information on site configuration options and examples from other communities.