Quality is delighting customers
You got some more details?
How did the devoloper know what to implement? Does the developer have any specifications?
My first thoughts & maybe some reference points for your tests:
(+) who designed & implemented the app? Talk to him/her and get background Information
(+) talk to the involved stakeholders (developer, customer,...)
(+) in which technology is the App implemented?
(+) are there any interfaces, databases, other apps involved?
(+) Are you dependend on any other system? (does an other system forward information to your system under test?)
(+) are there any restrictions to testdata?
(+) get to know the functionality
(+) dont forget performance (how many users- at once- will use the system?)
Important: I would write down all my conclusions in a extra document & use is as my specification.
This document should be updated permanently.
no problem - you'r welcome!
maybe an important poit I forgot to mention:
This "extra document", where you write down all informations you got (some kind of reverse engineerrd specification) should be available to every stakeholder, who also have to commit to this document!