You will need:
One department store
One friend in flat shoes (ideally someone with a better sense of dress than you)
Wear the underwear underneath your dress

Optional:
The shoes you will be wearing. Cocktail dresses are very short so any heel height will work. The shoes are essential if you’re using this method to make a full-length gown.

Method:
Enter department store.
Shop for ladies clothing.
Half-way dresses in your size are available. All colours, all fabrics. Don’t judge the clothes. It’s like brainstorming.
All your belongings should be brought to the changing room. There are changing rooms in department stores that look like they belong in a specific concession. They don’t. You can try any dress on in anyone’s changing area.
All dresses should be tried on with the correct underwear/shoes. Ask a friend to help you get the next size up or down.
Give anything that isn’t appropriate to a friend to give to the attendant.
Repeat steps 4-6 until you have tried on all the items and are left with a selection. It doesn’t matter if the dress fits well and doesn’t make your face look older or younger than it really is. It’s likely that you will wear it in a darkened area with drunk people.
Consider the event-specific criteria: Does it show too much leg/arm/cleavage Is it something that you can wear to multiple events? You shouldn’t reject something just because it needs to be cleaned. It is not a pair or jeans, but a cocktail dress. It will only be worn a few times per year and for a short time. It can be dry cleaned as often as you want. If you are invited to more events, you will obviously be in a different circle to me.
You can redress the mistakes of your selection by: a shrug, scarf, jacket, petticoat. A dress that is too short will not make you happy. Send a friend to get the items.
Add accessories to your look.
Limit your shortlist to 1.
Dress up and add accessories to your shopping cart.
Pay. Pay.