Theory:

Informal letter is a letter written to a friend or family members. It is used for non-official or personal purposes only. The language is informal and does not have many restrictions on the expressions used. However, there is a format in which informal letters are to be written.
Format of an informal letter:
  
Sender's address:
10, Prince Avenue
Nandanam,
Chennai - 600XXX
 
Date:
08th April 2024
 
Salutation:
Dear Raj
 
Body of the letter:
Introduction
 
Body
 
Conclusion
 
Subscription:
Yours lovingly
 
Signature:
Your name
 
Explanation of the format:
Address
The first thing to write is your address, i.e. the address of the writer, the sender’s address. We have to write the address on the left-hand side of the page at the very top. The address should be accurate and complete with the state and country mentioned. Even when writing to close friends or relatives, the address must be written, so that they can reply to the letter easily.
Example:
1. 25B, Model town,
New Delhi-110000,
India.
 
2. 45 A block, Piya Apartments,
Hyderabad-500012,
India.
Date
Next just below the address, we write the date. This allows the reader to have a reference as to when the informal letter was written. He can then relate better to the contents of the letter.
Example:
1. 25 Feb 2020
2. 30.10.2020
3. 14/11/2020
Greeting/Salutation
Now since you know the person you are writing to, the greeting can be informal as well. If it is a friend or someone close to your age you can greet them by their first name. If you are writing to your relative like your mother/father/aunt/uncle etc, you may greet them like "mom". And if you are writing to an elder person, someone you respect you can address them as Mr or Mrs.
Example:
1. Hi Roli
2. Dear Mom
3. Mrs Soni
Introductory Paragraph
Now we begin writing the actual letter. The introductory paragraph sets the topic for the whole letter. You might begin by asking the recipient about their well being. Or you may say that you hope the letter finds them in good health. The opening of informal letters should be casual and comforting.
Example:
1. It has been a while since I have heard from you. How have you been? I hope this letter finds you in the best of your health and cheerful spirits.
Body of the Letter
The letter overall, should maintain a friendly/good tone. But you have to adjust the language and the wordings according to who you are writing to. With a friend, you can afford to be casual. But if you are writing to an elder relative, you must be respectful and considerate. One way to determine the tonality of your letter is to remember how you talk to the person in a conversation. And then apply the same syntax and sentiments to the letter.
Example:
1. I recently came to know about your final year result. Your dad told me that you had gained the first rank in the entire school. You have once again made us all proud dear. This is a moment of elation and celebration. I congratulate you on your success. Your hard work has finally paid off.
 
2. The way you toiled day and night, I was already confident that you would come out with flying colours. Your success is an inspiration for your younger brother. He also wants to congratulate you in person. We are planning to visit your hostel. Please inform us about your schedule. And keep touching new heights of success. Our best wishes and blessings are always with you.
Conclusion
In the conclusive paragraph sum up the reason for writing the letter, i.e. summarize the letter. Say a meaningful and affectionate goodbye to the reader. And do not forget to invite the reader to write back or reply to your letter.
Example:
Hope to meet you soon.
Signature
To sign off informal letters, you can use words.
Example:
1. Lots of Love,
2. Best,
3. Best Wishes,
4. Kind Regards,
5. Kindly,

Pick the one that suits and then sign your name below.
Sample of an informal letter: Write an informal letter to your aunt, inquiring about her health due to recent illness.
Example:
10, Prince Avenue
Nandanam,
Chennai - 600158
 
08th April 2024
 
Dear Aunty
 
How are you doing? I heard you were not keeping well for the past two weeks. I'm very much worried about you. Please take care of your health and do not keep neglecting it.
 
Are you going for your doctor visits regularly? Please take all medicines at the proper time so that you can heal faster. Next month I have my school holidays, and I will surely visit you. We can roam around parks and beaches like we always do, but make sure you get better soon.
 
I miss you, aunty, hoping to see you soon.
 
Your loving niece,
Abinaya.