Please see pics below, I'll do my best to explain:
So this is how mine looked at the start, mine was an old crystal radio one:

The first thing I did was plug in the lower hull items as per the IBU2 Manual:

Manual can be downloaded here:
http://www.rctankelectronics.com/upload ... l_v5_2.pdf
Next I started on the turret, the first thing is a 3 pin connector, this is for the recoil and elevation:

This plugs in on the little board by the turret connector:

If yours was a 2.4ghz it should have a white socket for the wire to plug into, mine is an old board and I bodged it

The next bit is a 5pin wire from the turret that has heatshrink on it, this plugs into the socket on the board for Infra Red. If you don't use the IR battle feature you could leave this off. I also plugged in the other end of the 8 pin turret lead as you can see.

This is where things get a bit tricky. There should be a wire from the turret that looks like this:

The IBU doesn't need all 5 wires, the two with the extra plug are not required. This wire does the gun flash.
You can either extract the wires from the old plug, and fit them in a spare 3 pin plug, or you can cut the three wires and join the to a 3 pin wire.
The IBU2 manual explains how on page 16. The wires are held in by a tiny tab, you need to press this down with a tiny screwdriver or similar, then fit them in a new plug, after gently lifting the tabs up again so they lock in (I use a fingernail). I did mine this way, but you could be better off getting a new 3 pin cable and cutting and joining with block connectors as extracting the wires is fiddly.
Here's a 3-pin plug with its wires extracted:

Heres how it should look, the three wires have to be in the right order or it won't work.

This then plugs in like so:

The last wire should be a big red wire, with a metal joiner on the end. This is the earth for the recoil unit. Without it, the gun will fire but won't return to the start point.
I just cut the battery negative wire, strip the end of one side and inserted it into the connector:

Push this into a block connector, and tighten it down, then put the other half of the battery negative into the other side and tighten it all up:

I wrapped the block connector and wire in insulation tape so there's no risk of shorts.
Then you just need to plug the battery in and test everything works.

Hope this is useful,
Robert