Plugin usage help

I am new to serverless but not aws and having trouble with plugins

I have created a service and deployed it fine.

When I come to add a plugin to my service like serverless-crypt I install it in my service directory and add it to my serverless.yml file.
When I try to use it from the command line it is not found. I know I am missing somthing but have now been able to find out what.

Have you installed the npm package and added it your serverless.yml?