It's not clear what you mean by "complete" as that means different things during the checkout/order/fulfillment process
... XYproblem - What is the goal or example use case?
If by "complete" you mean fulfillment and need to learn to develop fulfillment apps then see the dev docs
https://shopify.dev/apps/fulfillment
If you are not a developer and you just need to automate the orders process use one of the automation apps available on the app store.
Mechanic is a go to for this https://apps.shopify.com/mechanic , if you want slightly more granular api control without having to build an entire app stack as it lets you script in liquid & json.
Example order automations https://tasks.mechanic.dev/?tags=Orders , https://tasks.mechanic.dev/?tags=Fulfillment
Goodluck!
