Okay I discovered there are fancy and fancier ways to do this, but here’s how I do it:
Half-fill a champagne flute with champagne, then top it up with orange juice.
I prefer dry champagne, but whatever takes your fancy. I never learnt to be a wine snob – don’t drink enough of it.
Here’s a fun non-snooty link: http://www.wineintro.com/champagne/basics.html