Remember Olympus is the mainnet, for most use cases this is the network you want to use. Only add Sparta if you know what you are doing and intend to use test POLIS (tPOLIS).
Just select the network you want to add, the website will prompt you to select the wallet of your preference, in this case it is Metamask, and confirm the network addition and switch.
If everything is correct, you should now be able of switching networks by clicking on the network selector on the top of Metamask. There should be a list of the available networks containing the Olympus or Sparta networks among Ethereum and other chains you may have added.
Manually Adding the Network
Here you will add the network parameters manually to Metamask, in case you want to verify you are connecting to the right network.
Tab on your account picture on the top right inside Metamask.
Go into Settings > Networks >Add Network
You will be requested to fill the network data, you will find it below for both Olympus Mainnet and Sparta Testnet.