Firstly you will need to identify if your POS system supports Zip, please see the full list of POS systems that have a direct integration with Zip.
If you are using an in-store POS system that supports Zip as a payment tender, you will be able to enable Zip and offer it to your customers at your register as an alternative payment method.
To activate Zip you will need to ensure your POS provider has enabled Zip on your account.
Once available you can activate Zip with your In-store API key and the relevant Location ID's for each store.