You need to create Wallet and Save it
Now here are some steps
Step 1 : Open Wallet and Click on top where you can see network click on it
Step 2 : Now you need to add new network Then click on add network you will see list of some networks
If You want to add one of then then just click on Add button so it will add if you want to add new token Then click on Custom Token
Step 3 : You will see form now you need to fill it with carefully all detail all networks have not same detail if you want to add core network then you can put this information and fill it
When you will fill it then click on save and go back you will see your new core network will add with your metamask Wallet and if you have Core tokens then your token will show in your balance