If you're unfamiliar with the location of your pelvic muscles, squeeze while you're peeing to stop urination midflow. You've successfully activated those muscles!
Once you've got that down, lie on your back with your knees up and shoulder width apart. Engage those muscles you worked earlier by flexing for five seconds, and then releasing for five seconds. Repeat this four or five times and repeat once a day until you're able to squeeze your pelvic muscles without needing to tighten the rest of your body.