Begin by understanding the PrestaShop database schema. This involves examining tables, columns, and relationships within the MySQL database used by PrestaShop. Access the database via a tool like phpMyAdmin or using MySQL command-line tools. Document the necessary tables and data fields you need to migrate.

Set up your PostgreSQL database. Install PostgreSQL on your server if you haven't already. Create a new database and tables that mirror the structure of the PrestaShop tables. Use the PostgreSQL `CREATE TABLE` statements to define tables with appropriate data types and constraints based on your PrestaShop schema analysis.

Export the required data from the PrestaShop MySQL database. You can use the `mysqldump` command to export data into a plain text SQL file or CSV format. For example, to export to a CSV, use a query like:
```sql
SELECT * INTO OUTFILE '/path/to/file.csv' FIELDS TERMINATED BY ',' ENCLOSED BY '"' LINES TERMINATED BY '\n' FROM your_table;
```
Ensure the data is exported in a format that is ready for consumption by PostgreSQL.

If necessary, transform the exported data to ensure compatibility with PostgreSQL. This may involve converting date formats, handling boolean values, or adjusting any MySQL-specific syntax that isn�t supported in PostgreSQL. You can use a scripting language like Python or shell scripts to automate these transformations.

Load the transformed data into PostgreSQL using the `COPY` command or `psql` tool. For CSV files, you can use:
```sql
COPY your_table FROM '/path/to/file.csv' DELIMITER ',' CSV HEADER;
```
Ensure that the data types and formats in the CSV match those expected by the PostgreSQL tables.

After importing, verify the data integrity in the PostgreSQL database. Run counts and checksums to compare the number of rows and data quality between the original PrestaShop tables and the new PostgreSQL tables. Write SQL queries to spot-check data consistency across key fields.

If periodic data transfers are required, automate the process using cron jobs or scheduled tasks. Write scripts to handle the export, transform, and import steps, ensuring they run at desired intervals or based on certain triggers. Test these scripts thoroughly to ensure they handle data consistently and efficiently.

By following these steps, you can effectively migrate data from PrestaShop to PostgreSQL without relying on third-party connectors or integrations.