If you were able to fine raw fresh Kathal, you can use below process to cut and clean it:
Clean the raw kathal with water. Place a paper on kitchen slab surface, apply some oil on your palm and on knife. First cut it in to three pieces cross section wise (basically three round pieces). Then cut all pieces in to half. Remove all the top green surface and then remove the hard part at the center. Also remove all big seeds. Drop all pieces of kathal in to warm water to avoid it getting discolored. Take out the pieces from warm water and clean it once before using for curry.