It's a very big ask, not as simply as just sticking a name down to make sure you get your flat. If you don't/cannot pay your rent for any reason (lose your job/get ill/the flat is a nightmare etc...) your aunt would then be liable to pay any rent you don't which could include notice period and, if in a fixed term tenancy like theusual initial 6 months, the entire rest of that period.
If via agents, they will often credit check guarantors (often with a fee for doing so) and ask for certain information such as copies of bank statements to prove the guarantor has the money to pay if you default.
The wording of the guarantor section would also need to be checked very carefully as there may be liabilities over and above rent.