Hi!
If you don't have 2 empty cassettes, just go to some dark place or in a changing bag to cut a 12exp 35mm roll and put the film in the camera, rolled at the giving end), pass it over the film plate area and close the back. You should be able to shoot it just for checking that the camera works fine. Don't forget to NOT open the camera until you're again in the dark though.
If you have the cassettes, loading them from fresh 35mm stock is not difficult at all. Just cut leader with some round shape, it helps to insert into the cassettes. The cameras I have (Agfa ISO something RAPID, Agfa Silette Rapid, ...) will work only with short films (12exp, or 16exp if the camera does 24x24mm) so I load them with 12exp rolls or with the equivalent length of bulk film.
The Agfa ISO Rapid are plenty and cheap to buy, just ask the seller if they come with at least one or two cassettes. I'm not 100% sure that these are the same as/compatible with the Karat ones though.
Luc