Here's how you add a custom token to MetaMask. This guide uses yUSD as an example but there is an easier way for yUSD and YFI with just a couple clicks at https://addyearn.finance.
1) Open MetaMask and click
Assets to see the tokens in your wallet.
2) Scroll down to the bottom and click
4) Paste the contract address for yUSD 0x5dbcF33D8c2E976c6b560249878e6F1491Bca25c into the
Token Contract Address field.
Edit to change the Token Symbol.
6) Type in yUSD and click
7) Hooray! Now you can see your balance of yUSD. Click
Add Tokens to finish up.
8) You're done. You'll now see yUSD in your assets list so it's easier to track and use.