Don't know how you mod your heads, but depending on whether they're hard or soft plastic, I have two methods I use:
I use dremel bits, drill bits, and a pin vise or dremel on low speed
If soft/flexible plastic: Use 1/8" drill bit and dremel bur bit (not sure on size (it's imperial), but it's just slightly larger than the neck ball)
1) Most important step for me -- measure for depth
a) Hold head up to torso to see how high/low it needs to sit
b) Hold each bit up to head and neck ball, so point is at the bottom of the head, marking where top of ball would be in the head
c) Wrap tape around mark (or use tape to make mark) to set the stop point. You can also use collars for this, but they seem to get in the way sometimes.
2) Use drill bit and drill to depth
3) Force bur into head to depth, then twist it around to hollow out the head to fit the neck ball
If hard plastic: use drill bit (again, don't remember size, but it's imperial vs metric and just slightly larger than the neck ball)
1) Measure for depth (same steps as #1 above, just with only the drill bit)
2) Drill to depth
3) (Optional, and I only do if the head seems too loose)
a) Fill freshly drilled hole full of hot glue or UV resin (Bondic* or other "As Seen On TV" resin pen)
b) Let glue cool down and solidify or hit resin with UV light for ~5 seconds
c) The head is now full of softer material, so go through steps 1-3 for soft plastic above to get proper fit
* I swear by this stuff now. It's handy to work with, and since it doesn't cure until exposed to a strong UV source, you have a long work time. It's become my go-to for attaching small pieces together, because I can line them up perfectly instead of playing 'beat the clock' trying to line up before a glue sets. If you look at my Deep Six custom, it's actually how I made his helmet, and it's how I gave Short Fuze his glasses.