Which advice would you give to someone who has just been diagnosed with Cold Urticaria?

Start taking Zertec and/or Zantec daily. If you go outside when it's less than around 55 degrees make sure you bundle up completely with as little skin showing as possible.

Join support groups and read up a lot. You need to be your own advocate with a disease this rare.

Be aware of your surroundings, wear clothing that keeps you warm and sweat free. Don't expose yourself to wind, make sure your air conditioned aware (most places have air conditioning during summer) be careful while swimming even in pools. Try not to eat cold foods or anything with ice.

Be aware
Listed. To advice but don't believe all of it
Be preparered
Be proactive

You can live with this. There are treatment options to reduce the severity of a reaction. You can do almost anything everyone else does.

See an allergist. Don't let your GP downplay your symptoms and wait for the possibiity of something bad happening. Not everyone has anaphylactic reactions but some do and until you are challenged, you're not going to know. I also wouldn't have worried anywhere near as much about taking such large doses of antihistamines, it improves my quality of life to do so by a huge amount.

Keep in mind that Cold Urticaria is rare, and it sounds like it came out of a fairy tale book. When you tell people, they won't always believe you, some will even call you a liar. There are support groups (theres a few support groups on Facebook) for Cold Urticaria and I would suggest to join one.
There are a lot of frustrations and confusion about Cold Urticaria that can be shared with support groups and you will find there are others with the same exact problem and knowing there are others with the same problems is comforting. A support group can always help you with learning the ropes with living with Cold Urticaria.

The most important details I can give you is to always carrying a jacket, because A/C is your enemy; the wind is also a problem and an antihistamine can do wonders.

Get advice from an allergist that knows about CU.

https://www.healthline.com/health/cold-allergy

Listen closely to your allergist or specialist and follow the treatment plan they lay out for you , and don't be afraid to let them know if something isn't working for you .