Let's talk for a minute about the whip-finish knot as used in fly tying. Many people are intimidated by this knot, but success here is simply a matter of practice. There are various tools available for tying the knot, but the best way is to learn to make it by hand.
There are at least three different ways to make a hand-tied whip-finish. One method, as illustrated in the accompanying video, is by wrapping thread with two fingers at the head of the fly. This is also the fastest method. I can tie a whip-finish knot this way in less than 10 seconds, although that comes after many years of doing it.
A very long time ago, and after years of using either a Thompson or Matarelli whip-finish tool, I realized I was spending too much darn time looking for the tool when I was ready to use it. So I resolved to learn the two-finger method, got a pro tier to show me how, and spent most of a day in dedicated practice until I could make it work. Once you do learn how, it's easy and--like riding a bicycle--is something you won't forget.
A second method is essentially the same as used to finish the thread wraps when installing a guide on a rod blank. Double a short length of tying thread to create a loop at one end (fine superline also works really well). Hold that section against the head of the fly with the loop extending to the right past the hook eye. Overwrap the looped section 3 to 5 times with tying thread. While holding the tying thread against the fly's head, cut the tying thread and put the tag end through the loop. Then pull on the loop's tag ends, which in turn pulls the tying thread back under itself to complete the whip-finish.
The third way involves doubling the tying thread back on itself, creating a loop and then overwrapping the thread with two hands like winding on tinsel or wool.
In all cases, when pulling a whip-finish tight, you'll need to use a dubbing needle or scissors point to keep a little tension on the loop as it is pulled closed. Otherwise the tying thread will kink and potentially snap as the knot is tightened.
I see an awful lot of misinformation these days when it comes to whip-finish directions. A half-hitch knot is not a whip-finish, to cite one common error. It places only one turn of thread as an overwrap as opposed to a whip-finish that is commonly 3 to 5 turns. So a half-hitch--or a series of stacked half-hitches--is nowhere near as secure. It works more or less, but not well.