Nobody seems to like their martini on the rocks. Don't be afraid, just prepare as usual; 3oz Gin, Beefeater is the classic, Bombay Sapphire,standard Bombay or Tanqueray will do, add a splash of vermouth, M & R or Noilly Pratt. Stir in a large glass filled with chipped ice. Strain into a seperate glass containing a few clear cubes. Add a few large olives or or if you prefer a few cocktail onions and call it a "Gibson".
The secret is to cut the gin with the chipped ice. Takes the harshness off the martini. I've been sippin on these for 35 years .
You should adjust the amount of vermouth according to you and your guest's preference. Of course serving "up" in a stemmed glass is a more than acceptible alternative, but me and my old martini buddies have always preferred "on the rocks".