Traumatic as it was, it sounds like you did just fine.

Just don't make the error I did the first time I gave Harmony fluids. I
forgot to shut the freaking valve off before withdrawing the needle,
resulting in fluid spraying all over the kitchen before I could turn the
darn thing off. Oops.

When the vet was showing me how to administer the fluids, she told me "and
then pet the cat for about ten minutes to make sure thatt the fluid gets
distributed". So I told Harmony, "See? She's prescribing petting. Vets
aren't all bad". (Harmony wasn't convinced. ;o) )

All the best to you and Cosmo. I'm sure that things will only get easier
as you get more accustomed to doing it, especially once Cosmo makes the
connection between getting fluids and feeling better.

Donna, Captain, and Stanley