The snail package is a MetaPost module within the ConTeXt LMTX environment, designed for creating flowcharts and similar procedural graphics.
Assuming you have properly installed the ConTeXt LMTX environment following the installation instructions on the ConTeXt wiki, and that your ConTeXt LMTX installation directory is /opt/context, proceed with the installation as follows:
$ git clone https://github.com/liyanrui/snail
$ cd snail
$ SNAILPATH=/opt/context/tex/texmf-local/tex/context/third/snail
$ mkdir -p $SNAILPATH
$ cp t-snail.mkxl $SNAIPATH
$ context --generateCurrently, only a Chinese guide is available at https://zhuanlan.zhihu.com/p/1938871262509273817
I'll prepare an English version when time permits.