Dear Wael Adil hereis a good description of how to carry our the Winkler method to determine dissolved oxygen in freshwater. It is available as full text on RG:
Method Winkler Method for DO Determination
Please also see this public full text entitled "DETERMINATION OF DISSOLVED OXYGEN BY IODOMETRIC WINKLER METHOD DATE OF EXPERIMENT":
Experiment Findings DETERMINATION OF DISSOLVED OXYGEN BY IODOMETRIC WINKLER METH...