More on strengthening the pelvic floor
The standard advice is to pretend as if you are passing gas. Women are sometimes advised to imagine that they are tightening the vagina around a tampon. In general, you should feel like you are pulling in the anal area.
It can be difficult to isolate the pelvic floor muscles. People contract their leg and abdominal muscles instead. It can help to put your hand on your belly so you can sense when you are contracting your abdominal muscles when you shouldn't be.