See also: How to Make Perfect Frozen Margaritas
Making the drink should be fairly simple. Of course, I've seen bartenders completely muck it up by doing a wild interpretation by adding all manner of ingredients, muddling things, pouring in extra spirits, what have you. What they make is all well and good, but it's gilding the lily.
To make a proper Pimm's, pour a shot of Pimm's into a glass, top it up with lemonade, and then garnish it either with restraint (a strawberry) or reckless abandon (everything in the bar garnish tray but the olives; I've been known to do up mine with orange, lemon, lime, mint sprig, a cucumber spear, and still had room to fit the strawberry).
There is one significant challenge in getting a right and proper Pimm's around these parts. It's the lemonade. If you order lemonade in Great Britain, you are more likely to get a sparkling lemon soda. Our lemonade doesn't have quite the same flavor profile as a commercial British lemonade, even if it's made with sparkling water instead of regular water.