There is only so much you can do to minimize the discomfort.
First NO DISTRACTIONS!
Cell phones, tablets, note books, laptops even TVs ALL HAVE OFF SWITCHES
DON'T BE AFRAID Use them!!
Fast for at least 12 hours so you are taking it on an empty stomach. Reduces nausea and essentially eliminates, on most enemas at least, any chance of vomiting!
Use WARM but not hot water. Hot water can raise the body temperature and make it uncomfortable and cold water is harder to retain.
Take a couple of minutes after you get in position to just relax before you insert the nozzle .
After the nozzle is in, take a couple of minutes again to relax, before you open the valve. If you need more than three deep breaths take more, it's OK!
Use a nozzle that is wide enough but NOT TOO WIDE, like anything else, comfort comes down to fit.
There will ALWAYS be some pain, it IS after all an ENEMA, but it does not need to be severe