Begin by exporting the data you need from Mailchimp. Log in to your Mailchimp account, navigate to the specific list or campaign you wish to export, and use the export tool to download the data in CSV format. This will provide you with a structured dataset that you can manipulate and load into ClickHouse.

Once you have the CSV file, inspect it for any inconsistencies or unnecessary columns. Clean the data by removing any duplicates or erroneous entries. Ensure the data types in your CSV file match the data types you plan to use in ClickHouse to avoid any issues during the import process.

Install and configure ClickHouse on your server if it's not already set up. Follow the official ClickHouse installation guide for your operating system. Once installed, ensure that ClickHouse server is running and you have access to the `clickhouse-client` tool for executing SQL commands.

Create a new table in ClickHouse that matches the structure of your CSV data. Use the `CREATE TABLE` SQL statement to define the table schema. Specify the appropriate data types for each column to ensure data is stored efficiently and queries run optimally.

```sql
CREATE TABLE mailchimp_data (
id UInt64,
email String,
first_name String,
last_name String,
signup_date DateTime
) ENGINE = MergeTree()
ORDER BY id;
```

While ClickHouse can directly import CSV files, it's beneficial to review and format your CSV to ensure compatibility. Ensure that the CSV uses the correct delimiter (typically a comma) and that any special characters within fields are properly escaped. Ensure there are no header rows unless you plan to skip them during import.

Use the `clickhouse-client` command-line tool to import the CSV data into ClickHouse. You can use the `--query` option to execute an `INSERT` command that reads from your CSV file.

```bash
clickhouse-client --query="INSERT INTO mailchimp_data FORMAT CSV" < path/to/your/data.csv
```

Ensure that `clickhouse-client` is pointed to the correct database and that the CSV file is accessible from the server where ClickHouse is running.

After the import process, verify that the data has been transferred correctly. Run queries to check the count of records, inspect a few sample records, and ensure no data is missing or misaligned. This step is crucial to ensure the data integrity and quality are maintained post-transfer.

```sql
SELECT COUNT(*) FROM mailchimp_data;
SELECT * FROM mailchimp_data LIMIT 10;
```

By following these steps, you can effectively transfer data from Mailchimp to ClickHouse without relying on third-party tools, maintaining full control over your data migration process.