Clio wiring loom

Hi, I'm a little new to all this forum business so please be patient with me! My husbands Clio has had intermittent electrical problems but the other day decided enough was enough. The dash board lights up like Christmas with every light possible on and that's it, nothing when you turn the key. I checked through a few possible reasons but all seemed ok. Whilst puzzling about it I discovered the worn wiring by the ecu plug. I have been looking through lots of posts regarding the tamper proof box rubbing through the wiring loom so now know it's a common problem. Unfortunately looking at the state of the wires there will be no way of repairing them so have sourced a new wiring loom. I understand that the Ecu may also need replacing but want to try this first. Does anyone have any tips on doing this? I've read about cleaning the contacts etc when swapping looms over, but is there a logical way of actually fitting the loom or is it just a case of patience and perseverance? Or am I biting off way more than I can chew! The Clio is a 1.2 51 plate and if this doesn't work it will be a gonna as it won't be worth spending much more on it.

Thanks for all the help so far! Jen