When using our bottles with a dropper top, wherever a recipe specifies "1 dash", use around 1 mL, or a full dropper (whatever amount is sucked up when you squeeze the bulb once). Technical measurements vary depending on who you ask, but a dash is around 0.8 mL. Our droppers are marked with incremental mL measurements for your convenience. Bonus: it makes you feel like a scientist or doctor when mixing up drinks.
If you're using a bottle with a dasher cap, invert the bottle over your drink and give the bottle a good hard shake, straight down. Each shake is 1 dash. Check out this video for a good example of how to properly "dash" bottles.