Go to Backup and Keys > Import by Currency.
Simply select the currency and paste the private key or mnemonic to the wallet you wish to import, then click Add wallet.
Alternatively, if you are creating a Guarda wallet from scratch, choose Import or Restore and you will be provided with the same option to pick a currency and enter the private key:
You can import multiple wallets in a row.
In that case, once the creation process is over, only the imported wallets will show up in your list of currencies.
Please note that certain encryption formats may not be supported.