You obviously can't go below the MDA (or circling minima, if that is necessary for your approach) or any stop altitudes that are detailed on the plate. If none you are quite allowed to descend to your MDA and maintain that until you reach the MAPt or you become visual with the runway.
In terms of lateral errors permitted you must maintain a QDM/QDR that is +-5 degrees of the designated approach. For example if the inbound QFU is 270 (all magnetic unless stated on plate) your needle on the RMI/RBI cannot deviate more than those 5 degrees either side. So it's your instruments that measure and show you this.