While you can cook it straight out of the vacuum-sealed packaging, quite a bit of trimming makes for the best presentation and ease of cooking. You'll find that one end is larger than the other. The large "Butt" end has a flap of meat that taste great, but should be trimmed for another use. The thin tip at the other end (good meat) should also be cut off and reserved for another use. These trimmings may amount to 25-30% of the total weight you purchase, but they make for succulent kebabs at a later date. Most stores when selling "tenderloin roasts" simply cut off the entire butt end and sell it separately. When the tip is also removed, you are left with what the French call a "Chateaubriand". It's also possible to buy these "center-cut" roasts directly from your butcher - but they are expensive. Properly trimmed, a Chateaubriand is typically 8-10" long, serving 4-6 people (I like slices 2" thick).
Also carefully remove the silver-skin (tendon) which is a slightly shiny thin piece of connective tissue running the length of the tenderloin. This is chewy and can either be removed by you at the start, or at the table by your guests. I choose the former. If there are large flaps of fat, remove these, but leave any small bits of fat you find, along with the strip of fatty meat that often runs along the back of the tenderloin. Trimming is easier when the meat is ice-cold, so do it straight from the refrigerator.
You should tie the roast. Get some butchers twine, and look up a video on YouTube. This helps to keep it uniform in shape (for even cooking and a nice round shape - rather than flat). It also keeps the roast from buckling when you brown it (twisting or curling). After all this is done, let it rest on the counter for an hour or two to warm up. Before cooking, pat the roast all over with paper towels to remove any surface moisture (makes for better browning) and season with kosher salt and fresh ground pepper.
Heat the oven to 375ºF and get a 10-12" oven-safe fry pan on high heat. Add a couple tablespoons of canola or vegetable oil (not olive) to the pan and wait until you see shimmering and a few wisps of smoke. Add the roast to the pan and be ready for spattering and some smoke (do not cover with anything, as it traps moisture). Wait at least 30 seconds, and then flip to another side - repeat on all sides. If you have tongs, things are made easier - especially holding it and browning the small ends. When brown, pop it in the oven.
This is where the most important part comes in - taking the temperature. A digital oven-safe thermometer that has an oven-safe probe is the best bet. You stick the whole probe into the small end of the roast, pull up the slack and close the oven door. Plug in the probe to the thermometer, and set it to 120ºF for Rare, 125ºF for Medium-Rare. Do not cut into the roast to check the color, or try to coax juices out - use a thermometer, and a good one at that.
When finished, remove the roast from the oven, out of the hot pan, and onto a cutting board away from the hot stove to rest for at least 15 minutes. If you did the trimming ahead of time, you just need to snip the twine off with some scissors, and cut slices as thick as you want.
The pan full of brown bits, juices, and rendered fat is also a gold-mine for a whole array of sauces that can be made while the beef rests.
Good luck.
Note - I recommend the OXO digital thermometer. It may seem a little pricey, but it's the first unit I've owned that has lasted more than six months.
