It's easy to add the Polygon network, you just need to follow the instructions below:

1. Connect or login to your Metamask wallet.
2. Click the Ethereum Mainnet dropdown menu as shown below.

3. Click “Add network” to go to the popular custom networks page.

4. Select Polygon Mainnet from the list of popular networks and then click “Add.”

5. Click “Approve” to allow the Polygon network to be used within MetaMask.

6. Once the network has been added successfully, click “Switch to Polygon Mainnet.”
7. Click “Got it” to continue.

You can now start selecting and buying NFTs on Polygon networks.