Hard to put into words, but practice semi-deep breaths. Breathe in through nose and hold breath for cpl of seconds. Release breath slowly through mouth (it helps if you make your mouth narrow, as if to whistle). When you are releasing, fight the urge to contract PC muscles. Fighting the urge to contract will produce small contractions.
Part of the process is developing the ability to detect the minuet feelings your pulsing prostate generates. Focus on the smallest of good feelings it brings and relax, let those feelings build and feel your prostate starting to swell.
Soon, you will be able to feel those contractions more at the base of your penis. Let that build, do not try to force it, but focus on that feeling.
Also, next time you ejaculate, pay attention to the position of yourself (in the context of pushing out and contracting your PC muscles). You don’t fully contract the upper portion. When you ejaculate the contractions are subtle and it feels as though you are pushing your prostate out through your perineum.