When developing Ember add-ons, you often want to test them in an existing project. Thankfully, the process is pretty easy:
With Yarn
ember install file:../path-to-addon
With npm
ember install ../path-to-addon
Because you are installing from a local path, ember-cli will not run the blueprints for you. As a final step, you will need to run ember g addon-name to complete the install process.