For technique have a look about 12 minutes into this video.
https://www.youtube.com/watch?v=aVUtdlzrktI
This guy is very, very serious about cleaning and really does know his stuff
.
Stoners Invisible Glass from here (other detailing retailers also).
https://www.shopnshine.co.uk/stoner
If your windscreen is truly filthy then mix some Barkeepers Friend (
https://www.johnlewis.com/bar-keepers-friend-stain-remover-multi-surface-cleaner-300ml/p3283874) with some "white" vinegar to make a paste then wipe the mix over the screen. Suggest covering the dash with a big towel or similar.
Buff off with lots of sheets of blue tissue roll (not a big handful turned around - use a new sheet and discard it, then another etc.). If it's not coming off just dampen a tissue with more vinegar.
Then finish off with a glass cleaner like Stoners or Autoglym.
Hope that helps.
Andy.