Istinja is the act of purifiying oneself from the physical (haqiqi) impurities (najas) after going to the toilet. The goal is to clean one's body and cloth from poo and urine to be able to pray.
Istinja can be performed by wiping with toilet paper or washing with water.
Water is preferable, yet toilet paper is also acceptable.
The optimal way is to first wipe with toilet paper, and then to wash with water three times (or more if needed).
Wiping and washing should be done with the left hand.
After urination, a man must ensure that no drops of urine remain in the urethra, which can be by squeezing the organ gently.
The aim is to ensure that no trace of wetness appears at the tip of the organ, otherwise, the remaining urine prevents the validity of one's wudu.
Women need only to remain still for a moment, and then may clean themselves (istinja).
It is important to do the cleaning in moderation, and not waste too much water or be obsessed with worry that the substance is still left on one's body or cloth after cleaning. Follow the recommended procedure, and then proceed with your wudu and prayer.