Howdy!
Off-hand open and pressed over the top to prevent jumpers, hold the mortar on a slight angle, with the pestle nestled between your thumb and forefinger.
Use a downward press with a twisting motion with your main hand, press and grind, press and grind. It seems to me it’s a numbers game, i.e. many strokes > fewer, harder strokes.
The magic is in the twist, not the press.
I use my M&P quite often, and I have a smooth one (big mistake!) So learning to do things like mustard seeds without making a mess was quite the adventure.