The first thing to do when working with pork tenderloin is removal of the silverskin. The silverskin is a tough grayish/white membrane that will not get tender during cooking. Go ahead and get rid of it now so you won’t be picking it out of your teeth later.
Start by sliding your knife under the thin end of the silverskin making sure the blade is parallel to the tenderloin. Tilt the blade upwards very slightly, and then gently slide the blade the length of the skin. Try to think of this as using your knife to peel the silverskin away rather than cutting it off.
Chances are that you will not get the entire membrane on the first attempt and that you also took some meat away with the skin. Go ahead and finish removing the membrane and don’t worry about the cosmetics. Nobody will ever notice if the tenderloin looks ugly on the bottom; everybody will notice if they have a chewy membrane in their mouth.
Now that the silverskin is gone you can start thinking about brines, marinades and rubs if your plan is to make grilled pork tenderloin. Alternatively you could do a little more knife work in preparation for stuffing or making medallions or cutlets.
If you want to stuff the tenderloin then it has to be opened up and pounded flat. Start by slicing the tenderloin lengthwise; you want your knife to go about two thirds of the way into the tenderloin but definitely not all the way through. Now you open up the meat, place it between two layers of plastic wrap, and gently pound it out to your desired thickness. This is something easier to learn by watching than reading. The first video from the National Pork Board shows how to make the initial cut and pound out the meat to a medium thickness. In the second video the meat has already been pounded out but I wanted you to see how flat you can get these things. These are two very different approaches to stuffed pork tenderloin.
Making medallions (for wrapping in bacon, etc) or cutlets is pretty straight forward. For medallions you will want them all to be about the same thickness so they will cook at equal rates. Start at the thick end of the tenderloin and cut off a medallion of your desired thickness (typically one inch thick is pretty handy). Now use that medallion as a guide for the remainder of your cuts and you should end up with medallions of identical thickness (the last one might be kind of ugly). To make cutlets for pan cooking you simply take a medallion and pound it out as shown above to your desired thickness.