Be calm, gentle, but firm. No build up, no "I'm sorry kitty, don't be mad" and try not to be tense. Just do it. (If you're tense, they're tense)

Use a treat or food to distract the cat, and hold them firmly (preferably, have a second pair of hands, but I manage just fine on my own)

Proceed with the processes, even if the cat complains, or gets a little feisty (back off if they're ballistic). Let them know (with your body language) that you're not stopping because they're unhappy

Also, make sure they have a good footing. Cats get uneasy if they don't have their feet on something, or if the ground is slippery (like a bathtub)

Put a towel in the bottom of the tub to give them better footing (not the best solution, but it works)