"Get the mail, Dorea."

Why did he even bother telling Dudley to do it? 

She goes to get the mail.

Three things lay on the doormat: a postcard from Uncle Vernon's sister Marge, who's vacationing on the Isle of Wight, a brown envelope that looks like a bill, and—a letter for Dorea:

Ms. D. Potter
The Cupboard under the Stairs
4 Privet Drive
Little Whinging
Surrey 

She stares at it for a moment--no one ever sends her letters--before quickly and surreptitiously slipping it under her clothes. She's not sure what Aunt Petunia and Uncle Vernon would do if they knew but she can guess she wouldn't like it. She dutifully walks back to the table and hands Uncle Vernon the bill and postcard.

No one spares her a glance.

"Marge's ill," Uncle Vernon informs Petunia. "Ate a funny whelk..."

Dorea escapes back to her cupboard as quickly as she can without looking suspicious.

The inside of the letter is just as fancy as the outside, written with green ink. The first page reads:

HOGWARTS SCHOOL
of WITCHCRAFT and WIZARDRY

Headmaster: Albus Dumbledore
(Order of Merlin, First Class, Grand Sorc., Chf. Warlock,
Supreme Mugwump, International Confed. of Wizards)

Dear Ms. Potter,
We are pleased to inform you that you have been accepted at Hogwarts School of Witchcraft and Wizardry. Please find enclosed a list of all necessary books and equipment.
Term begins on 1 September. We await your owl by no later than 31 July.

Yours sincerely,

Minerva McGonagall
Deputy Headmistress

Her heart leaps. 

After a moment she realizes it is obviously a practical joke or a promotion for something, and that's disappointing, but still, the fact that someone decided to send it to her is thrilling, and it is awfully fancy--

wait. 

If it's a practical joke, how do they know about the cupboard?

...Ee. 

What do they mean by owl, though? Maybe that'll be elucidated. She turns to read the rest.

Also in fancy ink:

HOGWARTS SCHOOL
of WITCHCRAFT and WIZARDRY

UNIFORM
First-year students will require

1. Three sets of plain work robes (black)
2. One plain pointed hat (black) for day wear
3. One pair of protective gloves (dragon hide or similar)
4. One winter cloak (black, with silver fastenings)

Please note that all pupils' clothes should carry name tags.

COURSE BOOKS
All students should have a copy of each of the following:
  The Standard Book of Spells (Grade 1) by Miranda Goshawk
  A History of Magic by Bathilda Bagshot
  Magical Theory by Adalbert Waffling
  A Beginner's Guide to Transfiguration by Emeric Switch
  One Thousand Magical Herbs and Fungi by Phyllida Spore
  Magical Drafts and Potions by Arsenius Jigger
  Fantastic Beasts and Where to Find Them by Newt Scamander
  The Dark Forces: A Guide to Self-Protection by Quentin Trimble

OTHER EQUIPMENT
  1 wand
  1 cauldron (pewter, standard size 2)
  1 set glass or crystal phials
  1 telescope
  1 set brass scales
Students may also bring, if they desire, an owl OR a cat OR a toad.

PARENTS ARE REMINDED THAT FIRST YEARS
ARE NOT ALLOWED THEIR OWN BROOMSTICK

Lucinda Thomsonicle-Pocus
Chief Attendant of Witchcraft Provisions

...This tells her absolutely nothing about how to get any of these things. 

Well. 

Alright. 

She composes a reply. 

To whom it may concern:

 I received your letter regarding my acceptance to your school, and would be happy to confirm attendance, if only I knew how. What do you mean by my owl, how am I to obtain the supplies, and how am I to get to the school? 

Sincerely, 

Dorea Potter.

Then she checks the envelope for a return address.

There isn't one.

Aargh. 

When she can get away with it she sneaks out of the house with both letters and enough filched spare change to pay postage and sneaks out to the post office.

"Good afternoon," says the person at the post office.

"Good afternoon. I got a letter and I want to reply to it but there's no return address, is there any way you can help me?"

"—no return address? I'm sorry, we can't send a letter unless we know where to."

"Okay. I don't know how the post office works, I was hoping you had records or something you could use. Thank you for your time."

"You're welcome." Smile.

She smiles back and leaves.

The Dursleys haven't really noticed she was gone.

Good. 

She is angry. Who do these people think they are, dangling hope in front of her nose and giving her absolutely no way to grab it--

She hides the letter--it's still fancy--and goes about life as normal.

Next morning there's mail again.

"Dorea, go fetch the mail."

She goes and fetches the mail.

The only thing there is is, in fact, the letter from the wizards.

...Um? 

Well, she already has the one, it's not like it'll hurt anything to let Uncle Vernon confiscate this one, and maybe he will have any idea what's going on. 

"There's a letter for me," she says, sounding surprised.

The best description of what Vernon Dursley does then might be "guffaw," but that probably does not do justice to the full-body experience that was. "Who would be addressing letters to you, girl?"