$ sudo apt-get update
$ sudo apt-get install fvwm-crystal 

fvwm-crystal is installed in your system.

Make ensure the fvwm-crystal package were installed using the commands given below,

$ sudo dpkg-query -l | grep fvwm-crystal *

You will get with fvwm-crystal package name, version, architecture and description in a table

fvwm-crystal package basic information: