Thread: How to design a web application by UML

It all depends on your (customers) needs and the application. UML is just one tool to design and document applications. So do you want to learn UML? Then I would suggest to just buy a book or something.

Question yourself the following:
- How do you want to use UML? Just for your own design process or for documentation and future use?
- How big is your project? Is it neccessary to document everything? Is it worth the effort? Find a balance in it.
- Is your application pretty straight forward or are there complex processes? Maybe you should only design the interactions of the complex parts.
- Are there multiple developers or project members at work that need the design documentation? Is UML a tool to communicate about the design within the team?
- Do you use UML to communicate with the customer you design the web application for? Does the customer want certain documentation? Does you customer understand UML?

UML has different diagrams for different purposes. If you, for example, want to model the shopping cart process of the website visitors then you can use an UML sequence diagram to model the sequence of activities. If you want to model the different actors and functionalities of the system (maybe together with your customer) and the dependencies between it then you can use Use Case diagrams to model this.

To make a long story short: first think about your needs and that of the customer and the goals of the design process and documentation of it. If you want to use UML to design your application: learn the different aspects and uses of UML. There is no single way to use UML. Just use it the way you need it in order to fulfil the goals of the design processes.