This could be the most useful thing you’ll learn when hand sewing.
Start by grasping the end of thread between your fore finger and thumb.
Wrap the thread around your finger, crossing it over the end of the thread.
It should still be grasped between your fore finger and thumb. Pinch higher up the thread between your middle finger and thumb and slide that finger and thumb down the thread pulling the twisted threads into a knot at the end of your thread.
The more you twist the threads the bigger the knot, if you need an even bigger knot wrap it around your finger twice. Practice makes perfect!
Once mastered it’s the quickest way of starting off your thread. You can conceal this knot on the inside of your work, under a hem or on the back of a seam and it never comes undone.