I prime with sugar cubes. Ones I use are Domino's Dots. They are equivalent to a half teaspoon of table sugar. These are also small enough to fit through most bottle neck openings.
One is perfect for a 500ml bottle. Drop it in, fill, cap. I put them in a bowl where I will be filling and drop the required number in the bottle just before filling.
These things are fast and easy.
I know some people that still just measure a half teaspoon of sugar per bottle and drop it in with a funnel.
All the Best,
D. White