I start with a general colour scheme! Then I know that the shoes/coats/accessories will go with multiple outfits. Even if it's a beach holiday I like my bikini/swimsuit to be able to double up as a vest top type thing.
Then I'll choose an outfit for daytime 'going out' depending on activity - hot place/sightseeing maybe a sundress for example.
Next some shorts and vest tops (not t-shirts cos I hate tan lines!) and a light jumper that goes with all the above.
Then a couple of dressier things if I'll be ' out out' at night, including a pair of heels that will go with both.
If I'm in the Uk walking or whatever it's just one pair of trousers/shorts (or shorts and some tights), couple of vest tops and a long sleeved layer, plus sweatshirt.
Lay it all out on the bed and see what can do double-duty and check any special underwear needed (eg Spanx or seamless or whatever)
I always also pack a bit of washing powder or liquid and plan to rinse things through if needed. I hate overpacking and over the years have realised that I often don't wear a lot of what I take, and I take the minimum.
Really, a hot beach holiday hardly needs anything apart from beachwear, shorts and vest and something to go out in!
A UK holiday just needs a couple more lightweight things that can be doubled up.
A weekend away can literally be done with just clean undies! But planning beforehand makes it so much easier!