Follow the steps described in this article Quickstart, followed by creating API Key here, then proceed to the next steps.
Go to the Credentials tab and create a new model using the Custom Engine option.
Make sure the model you selected has the status Available.
While on the Processing step, select the Google Glossary option.
You will see the list of available models, make sure to select the model you created while on Step 1.
Click on the model to configure the settings of the model created.
Go to the Cloud Storage on GCP and upload a glossary (CSV, UTF-8).
This file will have a URI like in the image below:
Go back to the Glossary feature to add the name and the URI of the file, then click Save.
The glossary will be added to the model in 30+ seconds, so give it some time to synchronize.