Begin by exporting your data from Gridly. Navigate to the relevant sheet or view within Gridly, and use the export functionality to download the data in a compatible format, such as CSV or Excel. This will serve as the source file for your data migration.

Ensure you have the necessary PostgreSQL client tools installed on your system. This typically includes `psql`, the command-line tool for interacting with PostgreSQL databases. These tools will allow you to create databases, tables, and execute queries directly.

Access your PostgreSQL instance using `psql` or any SQL client, and create a new database or connect to an existing one where you want to import the data. Use the command `CREATE DATABASE your_database_name;` if you need to create a new database.

Define the table structure in PostgreSQL that matches the data structure from Gridly. Use `CREATE TABLE` SQL commands to set up the tables with appropriate data types that correspond to your exported data. For instance:
```sql
CREATE TABLE gridly_data (
id SERIAL PRIMARY KEY,
column1 VARCHAR(255),
column2 INT,
column3 DATE
);
```

Open the exported file from Gridly in a spreadsheet editor or text editor and ensure that the data is formatted correctly for import into PostgreSQL. This may involve cleaning up headers, ensuring data types match your table schema, and saving the file in a CSV format if it isn't already.

Use the `COPY` command in PostgreSQL to import the data from the CSV file into your database table. This command allows for quick bulk import operations. Access your database using `psql` and run:
```sql
\COPY gridly_data (column1, column2, column3) FROM '/path/to/your/file.csv' WITH (FORMAT csv, HEADER true);
```
Replace `/path/to/your/file.csv` with the actual path to your CSV file, and adjust column names as necessary.

After importing, verify that the data has been correctly transferred. Run `SELECT` queries to check the data in your PostgreSQL tables and compare it with the original data from Gridly. Ensure that all records are accurate and there are no discrepancies or data loss.

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