If the fob you’re copying from is using the standard key, all you should need to do is have the existing fob on the hf antenna and then run:
hf iclass rdbl -b 7 --ki 0
Then take the valid fob off the proxmark and put the writable fob/card on and run:
hf iclass wrbl -b 7 -d XXXXXXXXXXXXXXXX --ki 0
Replace the Xs with the 8 bytes (16 hex symbols) that the proxmark3 output when you ran the rdbl command.
That should be it (assuming they’re not using non-standard ‘high security’ keys or something else going on).
Here is a list (incomplete and made very quickly) of posts that contain useful iClass information. I suggest you read each carefully as the next steps for standard and more involved iClass cloning have been covered elsewhere: