How do you make a great alfredo sauce?

The easiest way -

Melt a stick of butter in a saucepan, add cooked and well drained pasta to butter in pan, sprinkle 1/4 - 1/2 c. grated Parmesan (I use Pecorino Romano - fresh grated) and pour a bit of cream , half-and-half or whole milk; toss well and serve. (work fast - do not burn the butter)

I can't.

Seriously, this is one thing that I have not mastered. Mine always turns out lumpy.

have you tried melting the butter and adding the grated cheese and cream/milk - stir until smooth over low heat - but be sure not to boil it at all! then pour over pasta