How do you urinate?

The body has specific mechanisms for when the bladder will release urine, and how it releases urine.
First, the brain is always in a "go ahead and go" setting for urination. When the brain turns off that mechanism (like switching to a "don't go" or neutral setting), that is when we can urinate.

Second, the kidneys have been continuously filtering "fluids" from the cells, blood coming from the heart, and intestines. This is not just what we drink, but from physical processes in the body.

Third, the kidneys release urine into a holding area called the urinary bladder. It has a sphincter at the lowest part, right where it connects to the ureter (a tube that goes to the outside). At the other end, the ureter has a second sphincter. A sphincter is a muscle ring-like structure that keeps the tube closed. When the brain registers urinary bladder pressure, and turns the "always go" signal off, then the brain sends a message to the sphincters to open. But part of the sphincter opening is under voluntary control.

Each person is normally at the urinal / on the toilet when the sphincters relax so that we urinate. Sometimes this whole process goes haywire. For example, older men may have an enlarged prostate gland so they cannot urinate completely. Older women may be incontinent.