I spent a year doing that with an application, but had no manual. Exploratory testing is the name of the game. You pick a specific feature, like a calculating function or a screen control function or whatever, then you sit down with the application and work through that feature with intuitive process. Make lots of notes, maybe even outline a test case as you go. When you get to a drop down or data entry point, put in what you think would be a normal entry, then vary it and change characters to numbers and symbols to see if they are accepted, from this you can construct a set of requirements, given a whole lot of time, you will eventually map most of the application.
Ya thats the problem,we dont have a BA.I dont have options here,I need to do it.Whats the best way I can go about this? What way i can narrow down to my req's since evrything looks imp to me here.Any help appreciated!!