I can do it but I have to hold down the other fingers with my thumb or by pinching them into the palm of my hand.

Bend them the other way. Start with all fingers open for zero, and curl them as needed. You only need to move them a bit, so even twenty (thumb and ring finger back, the others curled) isn't too hard.

Try bending at the first finger joints instead of at the knuckles.

Flip your hand around

Then its a different finger

you can cheat it quite easily, just hover your hand over a table or surface, and touch your fingers to the surface to indicate a 1, and dont to indicate a zero, works on your leg, or someone elses, if you felt like it i guess.