its simple 
you will conduct black box testing  
step by step process

What do u mean by only PRD? If PRD is provided u can very 
well design ur test cases.

I will do requirement analysis use-cases
Identify gaps between requirement and use-cases
Develop high-level test plan
Develop test cases 
Develop tractability matrix between requirement and test 
cases to ensure adequat test coverage 
Execute test scripts and conduct various test like 
integration, system,functional,gui performance and lod 
testing

After getting the project requirement details we will first 
try to list out all the technical details and then prepare 
Architechture design for those specifications and Put it in 
to first low level design and then in to proper Code
After Coding is done we will do following
a)Unit testing --For testing code on module level
b)Integration testing --Once module are integrated to form 
system 
c)System Testing ---Once the subsystem are combined to from 
one end to end system or applicaion
d)Acceptance Testing --Finally we will do acceptance 
testing for the customer certification