Visual Paradigm Discussion Forum

Hello!
I am new to the forum..
I'd be grateful if someone here could give a solution to this problem.

I must make a communication diagram for the following case.
And after that an activity diagram of the same case.

1. The client comes with a product to return it
2. The cashier asks from the customer the receipt and he validates it.
3. The cashier starts in the register machine a new process for the return of the product
4. The cashier inserts the password of the receipt
5. The system takes from the store the Info that apply to the given receipt and displays the date and the total price
6. The cashier inserts the password of the product (that the customer brought to return)
7. The system search the linesale(sale products) and displays it.

Alternative flow:
5a. The receipt is not valid
1. The system validates that the product hasn't been purchased before 5 days
2. The cashier tells the customer about the problem and cancels the process

That was all.
It is very important for me. Thanks in advance.. Sorry for my English..

Here are some resources that may be helpful:

Communication Diagram Guidelines

Activity Diagram Guidelines

Is there anything in particular that you are struggling with?

thanks for the help, but i found those guidelines before and i couldn't solve this problem...
I have difficulty especially with collaboration diagram.
Thanks again..