I would not recommend bathing an older or unhealthy cat for the first time, as it can be really traumatic for them and might endanger their health.
Some things I’ve learned:
Keep the bathroom door closed. The cat will bolt immediately if it can.
Use a hand-wand sprayer, if you have one. I promise it will make everything easier.
Keep the water warm, but not hot. Also, keep the pressure on the low side. This will help keep your kitty from freaking out too much.
Avoid spraying their face. If you value your skin, you won’t do it.
Be gentle and talk in a soothing tone the entire time.
Have towels ready to rub them dry, and shut the doors to anywhere else in the house you don’t want them to get wet.
Consider giving them a treat after it’s all done to reinforce that the bath was not punishment.