Lay cone face down on table top
put top of voice coil onto voice coil opening of cone
use marker and drawer circle around outside of voice coil
(you can also mark the inside of the voice coil)
use a strait edge blade and cut cone inside the outer marked circle (if you marked inside and outside of voice coil, cut in between the two lines) be careful not to cut too large. it is easier to open the cone more if it is still too small.
use your thumb to gently smooth and enlarge the hole as needed
fit voice coil into new hole
Now that one cone is done you can use that cone as a template for the remaining cones and cut from that marking.