Get a helper, remove the drain screw at the bottom of each fork, do one side at a time, hold a container against the hole and SLOWLY compress the fork, do this until it is empty, then do the other side. BEWARE the oil can squirt out at a fair old whack so hold the container correctly and dont pump too hard.
Then measure the correct amount of the oil and put it back in the top having first replaced the drain plugs
The key is, to be sure they are empty before filling them, dont worry about air gaps.