$ sudo apt-get update
$ sudo apt-get install declarative-plasmoids 

declarative-plasmoids is installed in your system.

Make ensure the declarative-plasmoids package were installed using the commands given below,

$ sudo dpkg-query -l | grep declarative-plasmoids *

You will get with declarative-plasmoids package name, version, architecture and description in a table

declarative-plasmoids package basic information: