First make sure you have downloaded and setup metamask. If you need help with that, click here for a guide
a. Sometimes, you can visit a site thats compatible with the chain you want to add and it will automatically add it. For Polygon, you can try visiting https://quickswap.exchange/#/swap and press the "Switch To Matic" button. If you get a popup which says to add and switch network(like the image below) confirm it by pressing approve and you can skip the rest of the steps below.
On your browser's extensions button, press the metamask icon(the fox). If you don't see it, press the puzzle piece button to the right, and the pin on next to metamask.
Press the down button where the network is named. Here it is "Ethereum Mainnet".
Press "Custom RPC" to select a custom chain.
You will be presented with a screen to fill out. Use the details from the next step to fill them out.
For the name, use Matic Network, for the RPC Url use https://rpc-mainnet.maticvigil.com/, for the chain id use, 137. For the Currency Symbol use MATIC and for the block explorer use https://explorer-mainnet.maticvigil.com/. Once you have that inserted, press save.
Now you have Matic added in. You can switch to it by pressing on the network name at the top and picking it from the list. Most sites are smart enough that they can switch for you, but some require a manual switch.
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