If you believe that you are experienced enough to do this then do it by yourself. You have to migrate the data of customers, product, orders, etc. manually from your existing Shopify store to Magento.
It will initiate by exporting the CSV files for every data you require in your new Magento stores such as products, customers, and orders. When you get these files, you can import them on your Magento store.
From the Shopify admin page, select “Products,” and click “Export” above the product list.
A window of “Export products to CSV file” will pop-up. You can select All Products in the Export and CSV for Excel, Numbers, or other spreadsheet programs under Export as. After choosing all the options, click on Export products.
The exported CSV file will be e-mailed to the e-mail account which is linked with your Shopify store. You can import many different types such as Customers, Orders, Products, etc. You will require the CSV files for importing the data to your new Magento store.