Begin by familiarizing yourself with how data is structured within Ashby. Identify the tables, fields, and data types that are relevant to your transfer. This understanding will help you map these structures accurately to your PostgreSQL database.

Use Ashby's native export functionality to extract data. Typically, Ashby allows data exports in formats like CSV or JSON. Choose a format that best suits your data's complexity and your comfort level for processing it later.

Set up your PostgreSQL database if it is not already in place. This involves creating the database and necessary tables that mirror the structure of your data from Ashby. Ensure that the data types in PostgreSQL align closely with those in Ashby to avoid type mismatches during import.

Before importing data into PostgreSQL, clean and transform it as necessary. Use scripting languages like Python or command-line tools such as awk or sed to handle any data transformations needed, such as adjusting date formats, cleaning up strings, or normalizing data.

Utilize PostgreSQL's COPY command to import data directly from the CSV or JSON files into your database tables. For example, you can run a command like:
```
COPY table_name FROM '/path/to/exported_file.csv' WITH (FORMAT csv, HEADER true);
```
This command will efficiently load your data into PostgreSQL, provided the file paths and permissions are correctly set.

After loading the data, verify its integrity by running queries to check for completeness and accuracy. Compare row counts, perform spot checks on data entries, and ensure that there are no unexpected null values or discrepancies in the data.

Once the data is successfully loaded and verified, optimize the database by analyzing it and creating indexes as necessary. This step will improve query performance. Use commands like:
```
ANALYZE;
CREATE INDEX index_name ON table_name (column_name);
```
Tailor your indexes to the queries you expect to run most frequently for better performance.

By following these steps, you can efficiently transfer data from Ashby to PostgreSQL without relying on third-party tools or integrations.