Right need some pointers lads, now fitting a shower for my gran, doing the cabling side , not a problem.

Now in regards to the plumbing side, im a bit unsure of what to do , heres what i think lol

Now got 15mm flexi pipe, into loft area, chop into a water pipe for say cold taps for bathroom? insert a t piece?? in which bottom prt leads to my shower ??

now if this is correct? my pipes above i think will be old copper ones, and the flexi plastic stuff , does the t piece fit for this guys?

I realise shut off water and run water out realevent tap before chopping it, but dont want to cut it and realise i cant join it. also it says push fit connections for shower, what exactlys involved in connecting to rear of shower connector?

thanks for any tips help