You want to use TPOP’s print-on-demand service (you’re right) on your Shopify store? Just follow these few steps and in less than 5 minutes your Shopify store will be connected to our service, you’ll be able to “push” your TPOP products to it and we’ll take care of the rest (production, shipping, etc.).
The process is simple: in your Shopify dashboard, you just have to create a custom app, which will give you a key (a code) that you’ll simply paste into our platform. This key authorizes access to the functions needed to create products and receive your orders.
Summary:
Enable custom app development on Shopify
Create the app
Create the version
Install the app
Validate on TPOP
In your Shopify admin interface, click on Settings > Apps and sales channels.
Click on “Develop apps”.
Click on “Allow custom app development”.
Read the warning and the information provided, then confirm.
In your Shopify admin interface, then click on "Settings > Apps and sales channels".
Click on "Develop apps.
Click on “Develop apps in the Dev Dashboard".
Click on “Create app”.
In the area on the right, enter the name of the app (TPOP) and click on "Create".
You arrive on a new page “Create a version”. Go further down to the “Access” section and in the “Scopes” field, copy-paste the following list (containing the permissions the app needs): read_customers,write_customers,read_products,write_products,read_orders,write_orders,read_assigned_fulfillment_orders,write_assigned_fulfillment_orders,read_merchant_managed_fulfillment_orders,write_merchant_managed_fulfillment_orders,read_third_party_fulfillment_orders,write_third_party_fulfillment_orders,read_inventory,write_inventory,read_fulfillments,write_fulfillments,read_locations,write_locations
Click at the very bottom on “Release”. A window opens, click again on “Release”.

In the Dev dashboard menu on the left, click on “Home”.
On the right, click on “Install app”.
Information about the permissions you’re going to grant the app is displayed with a warning. This will allow TPOP to receive your orders. Click on “Install”.

Last step, go to your TPOP account, on the External stores page:
https://app.tpop.com/external-stores
Go to the Shopify line and click the “Connect” button.
Enter the URL (address) of your Shopify store (ending with “.myshopify.com”).
Go back to Shopify then to your Dev Dashboard (menu at the top right). In the menu on the left, click “Settings”. Finally, copy and paste the 2 keys (Client ID and Secret).

And just like that, you’re done: your store is now synced with TPOP. Products can be sent to your Shopify store and we’ll be able to automatically receive orders for these products so we can then ship them to your customers, it’s magic.
Official Shopify documentation: https://shopify.dev/docs/apps/build/dev-dashboard/create-apps-using-dev-dashboard