Sounds simple, doesn't it? Just Gin, and tonic water. What could possibly go wrong?
The devil, of course, is in the details. Here's my preferred details...
Use a short, chunky tumbler - a lo-ball glass - with a heavy base. Put in four or five ice cubes and let them chill the glass for a moment. Then, add a double measure of London Gin - I like Gordon's.
You should always add spirits to ice, and not the other way around, since this cools the spirits much more quickly. Add half a slice of lemon - no more, otherwise the taste of the gin will be obscured.
How much tonic water to add is very much a question of taste. But use a good-quality tonic - the cheap stuff from the supermarket should stay there. Personally, I like a long drink, so I usually fill the glass to near the brim.
Sip, and savour...