I have alfredo sauce, canned mushrooms, half and half, and shrimp (and maybe other things like pepper salt etc.) How do I make pasta cream sauce?

I would toss out the alfredo sauce for starters. Make a bechamel with some flour, butter and the half and half. Sauté the mushrooms and when they have thrown off all their water and are browned, add the shrimp for about 3 minutes. Then add the shrimpy mushrooms to the bechamel. Season to taste with salt and pepper. Voila!

Alfredo is already a cream sauce with cheese. It's a very poor match with shrimp.