Tips on Replacing old windows and doors

I have a project whereby I have a single backdoor standard size with windows either side (ears to the door)

I intend to remove the old metal crittal frames and then cut out the old wood frames that they were set into and remove those as well.

I will then replace in UPVC the door sourced from B& Q and the windows from a seperate supplier (60mm profile on each) and I intend to use a joining strip.

Ive never done this before so mildly terrified. Does anyone have any tips. Im particularly concerned about overhead support. I was working on the basis that there must be a lintel above (I can see a soldier course of bricks above the door window). Also unsure about setting the UPVC door sill/threshold (just screws and mastic?)

Any advice greatfully recieved

The soldier courses might be tied into a cast in situ concrete lintel with wall ties.
You should really go through Building control when replacing windows yourself.

I can suggest only one thing dear get bolt and screw from old windows and door. also a glass

My personal advice is that don't reuse any thing. Try to use new material because it is long life and also a reliable.