Largissimo – Someone I’ve read recently – I suspect it might be Van Gele – says that you should use whichever ‘words’ you find easiest. There are many, apparently. He did list some. But I expect Davis has them too. He also suggests practising them away from the recorder. I suppose that’s fine if you don’t mind people staring at you perplexed.
Or maybe instead of singing in the shower.
I’ve seen several writers on the recorder say that the double tonguing doesn’t come easily at first but it gets easier with time. They all suggest going slowly to start with. It’s kind of heartening, I think!