I will just add my 2 cents (Canadian currency,btw, therefore essentially valueless) as the previous answer seems to have basically covered everything, and note that I have found using my thumbnail to pinch the thumb hole (I am guessing that I really didn’t need to specify which hole the thumbnail might cover) seems to help me hit the high notes on recorders sized alto and down. Shading sometimes helps, but often is more hinders the process by throwing my intonation out of whack. Note: for tenor, I repeat process but add prayer or maybe light a candle. I should add that flesh adjacent to the nail is also used, just that the nail gives me a sharp edge to allow me to more easily leave just a tiny fraction of the hole open. As this will vary with player, I suspect, try fiddling about with it to find what gap works best for you.