On Guarda, you'll have to go to "Export by Currency" in "Backup by Keys", find BTH, then enter your password to see your private key. It's recommended that you save this private key somewhere secure if you're going to be transferring it, in order to prevent someone else from accessing this key.

You can then enter that private key into the Windows Wallet by running this command:

beth-cli.exe importprivkey "privatekey" "labelforkey" false

Replace "privatekey" with your private key you got back from Guarda, and replace "labelforkey" with a name of your choosing. The false option ensures that the import doesn't do a full rescan so make sure that's there or else you'll have to wait for a rescan of the entire blockchain which could take hours to days.