The Ryan Duns lessons are a very good start. Stick with it and you will progress well.
Here’s a handy way to refer to the fingers and holes on your whistle - the Top-hand can be either the left or right hand (usually the left) you can refer to them as T1, T2, and T3 (T3 as the ring-finger), the bottom hand can be named similarly - B1, B2 and B3. Most folks here will know what you are talking about if you use these terms.
There is no ultimate law concerning which fingers to use for cuts. However, there are advantages to learning the standard method with the index (T1) and ring (T3) fingers of the top hand. The first advantage is that it will sound like what you hear on recordings by experienced players. The second advantage concerns cheap whistles like the Generation D - a kind of double-negative really: In the second octave, the ring-finger cut using T3 on the D and E notes can cause a generation whistle to squeek if it’s not done crisply - this is a great discipline and teaches you how fast the articulations need to be. If the articulations are played too slowly, the melody gets compromised and the rhythm gets sloppy. Many of the high-end and expensive whistles don’t have this feature and can lead to a missunderstanding about how articulation works.
Part of the joy of whistling is the taming of the traditional cheap instrument ![]()
Hope this helps!