Mom found learning to express Mimosa to be a very frustrating experience.
I'm sorry you had a bad time with that one vet. I hope some of the urine ran off the table onto him.
Mom gets downright aggressive with vets if she doesn't like how they are doing things.
If you get him again, before you even take your cat out of the carrier you demand a towel, then if he is being rude you can point out that you are paying for his service, not his attitude thank you and if he woudl be helpful then you could maybe learn something.
Mom did actually learn from her vet. Tehre are picts of her expressing Mimosa on teh instruction page.
Pretty much mom kept trying and trying to catch Mimosa's bladder against her pelvis but it is slippery and it kept slipping out one way or the other, and Mimosa knew just how to kick to make it pop out of mom's hand.
then one day, magically, it worked.
and it worked again. mom was thrilled.
she has been expressing Mimosa 2 x a day for almost 7 years now.
we've read a lot of similar experience, that it just suddenly works, like your fingers finally find the magic way to press and TADAH it works.
it is good she is starting to pee on her own, Mimosa will sometimes, but it is important to empty out that bladder as much as possible
hey, here is a thought. practice aiming the stream and then take her in to the snotty vet and say "oh let me try" and squirt him with it, then do a little happy dance saying "it worked! it worked!"