Here are two ways to perform negative-accentuated lying leg curls.
Method #1
Method #2
The key is to lower slowly for a count of 8 seconds in a smooth and uniform fashion from the top to the bottom. If it gets to a point where you cannot control the movement and it lowers too quickly (under 6 seconds), stop the set.
Learn more in Eccentric Training: How to Obtain Positive Results From a Negative Action.