Private pilot license has limitations, one of them is flying during low visibility conditions under instrument flight rules.
To obtain instrument rating you need to be in possession of a valid Class 2 Medical certificate and be holder of a valid private pilot license. by the time of the skill test, you should have 50 dual hours and your ATPL theory and must demonstrate the ability to read, speak, write and understand the English language.
Testing consists of a practical Flight Test. The Flight Test is divided into an oral component to verify that the applicant understands the theory of instrument flying and an actual flight to ensure the pilot possesses the practical skills required for safe IFR flight.