Shopping thing: you have to figure out which beef roasts only work as pot roasts and which work as oven roasts. Pot roasts go in a pot with liquids and a lid on top to help them stay moist and become tender enough to chew. Oven roasts are the kind that can be cooked pretty much dry in an open pan because they're more expensive meat that won't dry and be tough that way. At Danville Kroger, the roasts just say "oven roast" or "pot roast" on them, so you know. If you're elsewhere and don't have hints that obvious available, ask a butcher or someone standing next to you. Or call someone who reminds you of Hermione and ask that person to look the information up on line.
An oven roast (pretty much any size)
Chinese hot mustard (My no-longer-secret ingredient)
A baking pan--like one you'd use for brownies or a cake, with sides
A meat thermometer (or a knife to cut the meat open and look at it)
Multiply the pounds of your roast by 20 minutes a pound, and then add 20 more minutes. That's how long it will cook.
Think the timing backward, from when you want to eat, to when the cooking time starts, and then back another hour.
That hour ahead, take the roast out of the fridge and put it on the counter to start warming.
45 minutes later, start preheating the oven to 425 degrees.
Unwrap the roast and put it in the pan.
With a knife, take a big dollop of mustard out of the jar and flick it onto the roast. Put the knife down so you can use it to get more mustard if you need it.
Use your hands (yes, your hands) to smear the mustard all over the roast, very thin.
Stick the roast into the oven for the pound x 20 minutes.
Then check the meat. With a meat thermometer, stick it right into the middle of the roast (and only halfway through) and check that it's at least 140 degrees. Without a meat thermometer, take the roast out of the stove, cut it in the middle, and peek to see if it looks done enough.
If it isn't hot enough/pink-to-brown enough, stick it back in the oven for another 10 minutes and recheck.
When it is hot enough/pink-to-brown enough, take it out of the oven and let it sit for five minutes or more before you slice it.
Nigella Lawson's How to Eat taught me most of this, except she thinks you should use English mustard and like the meat rare when it's as cool as 120 degrees.