Other person to cover the cats eyes as if they can't see, less panic - put cotton wool in ears to reduce risk of water getting in and block out sounds
Hot water, I use the electric shower turned at one notch above what I use for myself as cats body temperature is a degree higher than ours and it helps to stop them getting chilled
Towel down so they have something to grip into as otherwise sliding around, so more secure - don't let the water build up under the paws as cats brain says that they are being drowned so panic scramble to get out same as we would if the same was happening to us
Hold the shower hose right up to the fur so there is no spray from it, if held close to the body they are wet before they even register it especially if eyes are covered. Rinse the shampoo well, don't leave any residue as it can cause irritation
Wet a ring around the collar area and don't let above the ring get wet, this also helps in case of any fleas as they will try to run up and get drowned effectively from the collar area down. I wash around the face with a wet facecloth
Squeeze as much water off them before you take out of the shower, tail, paws and wrap in towels to dry off
Be prepared to be ignored for the day for this assault, dispense treats and allow lots of ignoring and licking to get their cat smell back into themselves. Keep inside and warm