TEACHING YOUR DOG TO LOVE HAVING A BATH/SHOWER

Encourage you dog into the bathroom using food and/or toys.
Have the shower cubicle door open and make sure that it is left open, even when your dog is in there. It is important that your dog knows that it can make the choice to come out if and when he has had enough. (remember that we are not using water just now)
Have the lickie mat primed and on the wall of the shower cubicle at the right height for your dog to be able to access it easily.
Toss some treats on the bathroom floor and have your dog happy about being in the bathroom.
Making sure there is rubber matting in the shower tray so that the dog does not slip, toss some treats into the shower tray.
Once your dog is happily going into the shower tray and is accessing the lickiemat you can open the bottle of shampoo and allow the dog to sniff at it. Any attention your dog pays to that bottle of shampoo should be rewarded.
Take the showerhead and lay it on the shower tray but DO NOT TURN IT ON! Scatter treats all over the shower tray and allow your dog to eat these. Recharge the lickiemat if required.
Have a small jug with hand warm water in and gently pour a little onto the floor of the shower tray, making sure that a small amount can touch your dog's feet. Feed treats
try gently drizzling a little water up you dogs legs. Feed treats
try gently drizzling a little water over your dog's back end and tail area. Feed treats
Eventually working your way up to wetting your dog all over with the exception of his head, with water from your jug BEFORE thinking about turning on the shower. Most dogs dont like having their head wet or having water running down their faces, so this would be the very last place to wet – having made sure that your dog is 100% about being showered everywhere else first!
Once your dog is totally happy being wetted from the jug, you can turn the shower on but very slowly. We only want a trickle coming out. Feed treats.
Slowly build up the pressure that you have the shower on. Feed treats.

You can see how slowly and carefully you need to introduce the whole shower/bath thing to a dog. Your dog must be absolutely fine and showing no signs of fear, anxiety or stress (FAS) before moving onto the next step.