D.W.
Never marinate a filet! Use kosher salt and fresh coarsely ground pepper. Put a small amount of butter the pan once it's hot and then put the steaks in. Depending on the thickness cook for approx 2-4 minutes each side. The steak will still have some give if you push on it but it won't be too firm and it won't be too soft for a medium-medium rare. Just watch it closely if you aren't used to cooking filet.
After they are done let them rest on a plate for 5 minutes to allow the juices to redistribute into the meat. If you cut them too soon the juices just run right out.