First, ensure that you have both Python and PostgreSQL installed on your system. You will need Python to read and parse the JSON file, and PostgreSQL to store the data. Verify the installations by running `python --version` and `psql --version` in your terminal.

Open the PostgreSQL command-line interface by typing `psql` in your terminal. Create a new database by executing:
```sql
CREATE DATABASE your_database_name;
```
Replace `your_database_name` with your preferred database name. Then, connect to your new database using:
```sql
\c your_database_name;
```

Determine the structure of your JSON data and create corresponding tables in your PostgreSQL database. For example, if your JSON file contains data about users, you might create a table like this:
```sql
CREATE TABLE users (
id SERIAL PRIMARY KEY,
name VARCHAR(100),
email VARCHAR(100),
age INTEGER
);
```
Modify the table structure to match the schema of your JSON data.

Use Python to read your JSON file. This step involves opening the file and loading its contents into a Python dictionary or list for processing:
```python
import json

with open('data.json', 'r') as file:
data = json.load(file)
```
Replace `'data.json'` with the path to your JSON file.

Use the `psycopg2` library to establish a connection to your PostgreSQL database. First, install the library using `pip install psycopg2-binary`. Then, create a connection and cursor:
```python
import psycopg2

conn = psycopg2.connect(
dbname="your_database_name",
user="your_username",
password="your_password",
host="localhost"
)
cursor = conn.cursor()
```
Replace the connection parameters with your database credentials.

Iterate over your parsed JSON data and insert it into the PostgreSQL table. Construct an `INSERT` SQL query and execute it using the cursor:
```python
for item in data:
cursor.execute("""
INSERT INTO users (name, email, age) VALUES (%s, %s, %s)
""", (item['name'], item['email'], item['age']))
```
Adapt the query and field names to match your JSON structure and PostgreSQL table.

After inserting all the data, commit the transaction to save changes to the database and close the database connection:
```python
conn.commit()
cursor.close()
conn.close()
```
Committing ensures that all your data modifications are stored, and closing the connections frees up resources.

By following these steps, you can successfully transfer data from a JSON file to a PostgreSQL database using Python without relying on third-party connectors or integrations.