Take requirements
What do we need to implement?
Why? Which is the objetive?
This is for a Prototype, Demo, Production Development, Post-Production Development?
Which results I must delivery?
Find referents
Try to find other video games where the feature has been implemented.
Make the Approach
Concept
Understand the requirements crearly.
Understand how the module work.
Identify every element of the module.
Use the correct naming for every element.
Which elements need animations?
Which elements need text localizations?
Do you need research and make tests?
Architecture
Consider to divide the screen in modules if the screen is complex.
Define Architecture Patterns that you need to use.
Controller
View
Containers
Object Pooling
Factory
Services
Prefabs
Define how much scripts you need.
Define the responsibilities for each script.
Define what kind of classes you need for implement the controller.
Unity
Make the Screen Prefab
Consider if the screen contain containers.
Consider if the screen contains Items or Elements.
Consider the Notch in some devices.
Consider the multiples resolutions.
Consider the lenght of the texts and its containers.
Code
Consider if the module has sub modules:
Consider the view
Consider the controller
Consider the sub prefab if this is necessary.
Consider if the module has or need special components.
Main View Script (Animations, UI Components, Items Creation, Others)
Inheritance (AbstractPopupView)
Fields (UI elements)
Methods (Initialization, Interact with UI elements)
Main Controller Script (Base class)
Inheritance (AbstractPopupCont)
Fields (View)
Methods (Awake, Any Other)
Others Dependencies (Services, Others)
Test feature, you could create an example Script and Scene.
Make the estimations
Validate that the feature has been approved
Make the Prefab and Example Scene (Place Holder)
Make the functionality (Programming)
Test the Functionality (Example Script & Example Scene)
Send Pull Request (Technical Review)
Present Results (Client Review)
Integrate the feature into the target branch