Remove the handle screws and drill out the hole to 6mm.
Screw the eye-bolt through the hole. If you're right handed you want the eye on the left side and if you're left handed you want it on the right.
Once you have tighten the bolt up use a hacksaw to cut the bolt down flush and tidy it up with a file.
Now it's time to rig up your spear. Attach the shooting line to your spear as normal and run it back over the line release.
Instead of attaching it at the muzzle you make another half-wrap and run it back to the handle.
Make the loop in your shooting line even with the loop from the toggle.
Use a larks foot to attach the toggle to the shooting line leaving the loop free to clip your floatline into.