Can i tint the rear side windows on my car?

You need to take it to a proper registered window tinter, they will give you the paperwork legalising it and put a sticker behind the tint. There's no need to change the Ficha Technica, just make sure the paperwork is it hand at ITV time.

In my experience I've never been asked for the paperwork because they can see the sticker behind the tint.

That's great, thank you. I had read that you have to have it done professionally but wasn't totally sure what that entailed, and if it was correct

I'm not sure that's the case.

A little out of date but N332 make mention of a change in the law.

https://n332.es/can-i-tint-my-windows/

One problem with DIY is that it can be  difficult to get the glass clean enough which can result in peeling and/or bubbling (we've all seen it) and that can lead to an ITV fail.

I have tinted windows but they were done professionally so I have a certificate for them but in 5 years have never been asked to show it. I have NO sticker behind the tint or anywhere else.

Caveat Emptor though, YMMV! (Your 'Mileage' Might Vary)

We got ours done at autoglass (UK's Carglass), fantastic service 10year guarantee at any of their garages. Includes the stamp for ITV  and police checks

Thank you, think we may have decided just to stick with the window blinds

Will look into that, but short on time so may just stick to window blinds

It took only 5hours

The Law change in the link you provided clearly states you no longer have to take it to be approved at the ITV station and have the changes noted on the Ficha Technica (which I mentioned). It still needs to be done by an approved company with approved tinting film.

The rules may have changed since you had them done 5 years ago as they now need a small approval sticker between the tint and the window, it's only approximately 10mm in diameter, you have been lucky or had lazy testers if you've never been asked for the paperwork and don't have the approved sticker.

DIY is definitely not allowed, but as with everything, it's down to everyone to make their own decisions and do as they feel best for them. I chose not to take the risk of them seeing non approved window tints, and have them look closer to see if anything else was amiss.

Thank you for all who replied, you have been very helpful