My favorite drill for lag putting is the “ladder drill” where you putt to the different squares (like the image below). For specifically lag putting I suggest putting to the squares that are farther away. Constantly changing which square you putt to is going to make you the best player. Being able to work your way up and down the ladder will help you get a feel for the green and your stroke.
For an added challenge, make the squares narrow and compact to help not only the speed, but the direction.