This FAQ picks up the process on the Select Part screen. There is also a document that explains it in detail. If you need that document, see the link below.
When you're choosing your part need, you'll be offered options that will be similar in form/fit/function. You might want to note which one(s) are in stock and which one(s) match the best.
Note how many are available. This will help you on the next screen.
Enter your quantity needed.
Click the Add to Task button. You will have the ability to modify it on the next screen.
This screen can seem confusing at first but it's really not. Think of it this way. The basic idea is to help the technician get the right part on the machine in the most efficient manner.