Basically - take your favorite pumpkin pie recipe and substitute an equal amount of cooked (baked) and mashed sweet potatoes for the probably canned pumpkin called for in the recipe.
If you used canned pumpkin (like most people) - you probably will not notice any difference if you use sweet potatoes. If you use fresh pumpkin you might notice some difference - but the taste will still be similar.
I only use about half the amt of milk or evap milk frequently called for in a pumpkin pie recipe. I like the flavor of the pumpkin best. One I use is:
4 C pumpkin well dried by putting canned pump. in a pan to cook to give a bit of carmelizing to it.
2 C milk or lt cream
spices and salt