Melt butter slowly over low heat in a large or medium skillet.
Once the butter is melted, start adding the shrimp to the pan so that it is in a single layer and well emersed in the butter. Slowly cook the shrimp over low heat for about 5-7 minutes then flip them to the other side for another 3 minutes. You don't want to overcook shrimp, they will get a rubbery texture so be sure to watch for the telltale signs that the shrimp are done.
The shrimp are done when the internal temperature reaches 125°F or when the translucent flesh becomes opaque at the thickest part of the shrimp.
If you like the flavor of browned butter (I do), when you melt the butter, melt it over medium heat until it starts to brown then turn the heat down to low to cook the shrimp. This will add a richer butter flavor to the poached shrimp.