Begin by exporting the data from Notion. Navigate to the Notion page or database you wish to export. Click on the three dots in the upper right corner, select "Export," choose the format you prefer (such as CSV for tables), and save the file to your local machine.

Ensure you have a working PostgreSQL server installed and running on your local machine or accessible from your network. Also, make sure you have a PostgreSQL client (such as `psql`) or a GUI tool (like pgAdmin) to interact with your database.

Access your PostgreSQL server using your preferred method (CLI or GUI). Create a new database by executing a command like `CREATE DATABASE notion_data;`. Once the database is created, switch to it using `\c notion_data` or equivalent in your GUI tool. Define the schema and create a table structure that matches the data format you exported from Notion. For example:
```sql
CREATE TABLE notion_table (
id SERIAL PRIMARY KEY,
column1 TEXT,
column2 TEXT,
...
);
```

Open the exported CSV file from Notion using a text editor or spreadsheet software. Ensure the data is clean and correctly formatted for PostgreSQL import. Remove any unnecessary columns or rows, and make sure that date formats, numbers, and text align with your PostgreSQL table schema.

Use the `COPY` command or the `\copy` command in `psql` to import the data into your PostgreSQL table. The `COPY` command is executed on the server, while `\copy` is executed from the client. Here’s an example using `\copy`:
```sql
\copy notion_table(column1, column2, ...) FROM '/path/to/exported_file.csv' DELIMITER ',' CSV HEADER;
```

After importing the data, validate it to ensure everything transferred correctly. Execute SQL queries to count the rows and check for discrepancies. For instance:
```sql
SELECT COUNT(*) FROM notion_table;
```
Compare this with the number of rows in your CSV file. Also, perform spot checks on random rows to verify data integrity.

If you need to perform this transfer regularly, consider writing a script in a language such as Python or Bash to automate the process. Use libraries like `psycopg2` for Python to interact with PostgreSQL and automate CSV reading and data insertion. This will save time and reduce human error in future data transfers.
By following these steps, you will have successfully transferred data from Notion to PostgreSQL without relying on third-party connectors or integrations.