Start by prepping the spinach. The best way to cut spinach is to hold the spinach bunch gently by the leaves and cut the thick stems at their base in one go. Discard these. Don't worry if there are some thinner tender stalks still left. You can leave the leaves whole or cut them as per the length you want.  

Wash and clean the leaves. Pat them dry with a kitchen towel. Alternatively, you can also use a salad spinner. 

Melt the butter or heat the olive oil in a wide pan or wok over medium heat. If using butter, take care that it does not burn or brown. 

Add the minced garlic and saute them lightly. Add the spinach just as the garlic is beginning to brown.

Saute the spinach in the garlicky butter or oil with a pair of tongs or a spatula. The spinach should just wilt. This will take only about a couple of minutes. Do not overcook the greens. 

Make sure that the spinach is in one even layer. If your pan is too small to take all the leaves in one go, cook in batches. Cook as much as it can take, wait for it to just wilt and then add some more. Try to finish this process in two parts and avoid overcooking the spinach. 

Your delicious sauteed spinach is ready!