You've created a package that suits your needs. And now, you want to use it on your project.
You have two options. Install the package direcly or link it. I personally recommend link it. Because this way every time you update the package, it will be updated in all projects you use the package. Otherwise you will have to manually update the package.
Open your npm package project folder on terminal, and run:
Next open the project folder you want to install the package, and run:
npm link package-name
Where "package-name" is the name of your package.
If you want to install the package, it is very simple. Open the project folder (that you want to install the package) in the terminal and run:
npm install /path/to-your-package