A few things to look out for At the rear of the engine there is a brace which you need to remove.This needs to be bolted back on over the underpanel ie lift the underpanel into position then put the brace on.You will need to put two flat washers between the K frame and the underpanel because that mount is slightly higher than the other two,
Next the plastic panels on the side that bolt into the underpanel have about a 25mm gap between them,you can stick you hand in from the back and gently push them down to get the bolts in.It looks ugly to start with but they tighten up OK
When you get all the bolts in (a bit fiddley) Tighten the 3 rear 8mm bolts at the rear first.This gives you a good straight starting point to align everything else.Next tighten 4 of the 6mm bolts at the centre front then pick one side and start to tighten up from the front centre around towards the wheels.Finally tighten the 6mm bolts that bring the underpanel and the side plastic panels together.Take your time and keep adjusting as you go and you will end up with a pretty good fit.
One other thing I just thought of make sure when you bolt the two halves together fully tighten the bolts BEFORE you fit the underpanel they arehard to get at when it is in place.