Since you got the assignment, time is an essence since you got 6 months.
Start with the domain model, understand that make that part in the class diagram including the relationships.
Then go to the use cases, go through them get the similarities and differences. Then choose one of them and drill down the detail from web/presentation -> business -> integration tiers. As you do this for each use case, add them to the class diagram and make that sequence diagram. The more detail the flow, the better it is for you to see to develop it (if that was the case).
Repeat this step for X use cases and after the first round,
you should have a decent class diagram AND sequence diagrams.
Oh while doing all that, document your risks and assumptions, and other things eg
patterns, NFR in your HTMLs.
After the initial round, review it to see if it makes sense and clear. If not enhance it accordingly.
When you are happy with the class and sequence diagrams, do the component diagram which is a higher-level diagram of the class diagram.
Good luck and happy drawing.
Lastly the deployment diagram taking into those NFR like availability & scalability.