Tying a roast or a joint is a useful technique that helps it retain a nice shape as it cooks, which leads to both better presentation and more even cooking. Even if you cook a lot of meat, it's very easy to get through life never knowing how to tie a butcher's knot.
I used to use regular old square knots to tie up roasts. But butcher's knots have an advantage: They're slip knots, which means that once you tie them, you can adjust them very easily without needing an extra finger to hold the knot in place as you tighten it.
I like to use 100% cotton twine because it grips the meat nicely as you're tightening and won't melt or burn in the oven.
Here's how to do it, both in video form, and as a step-by-step photo series.
And now step-by-step. For all of these photos, I'm assuming that you are right handed. Lefties may want to reverse everything.
Step 1: Slide Twine Underneath Roast
Place the roast parallel to the edge of your work surface and set the roll of butcher's twine on the work surface near you. Slide the butcher's twine (still connected to the spool) underneath the roast so that the cut end is on the far side of the roast.
Step 2: Bring the Far End of Twine Towards You
When you have the twine in the appropriate position (roasts are generally tied at about one-inch intervals), lift the far end of twine over the roast and towards you so the twine is wrapped around it.
Step 3: Arrange Cut End on the Left
Place your left hand under both sections of the twine, laying the cut end of the twine to the left of the other end.
Step 4: Pinch and Lift
Pinch both ends of the twine between the forefinger and thumb of your left hand, then lift up the cut end with your right hand.
Step 5: Wrap Around Your Thumb
Lay the cut end over your left thumb, letting the end fall to the left.
Step 6: Bring it Under
Reach under both pieces of twine with your right hand, grab the end that is dangling down, and pass it underneath to the right. There should now be a pretty tight loop formed around your left thumb.
Step 7: Lift and Thread Through the Hole
Lift the end of the twine up, bring it over to the left, then pass it through the loop wrapped around your thumb. As you pass it through, make sure that it goes through the loop in the same direction that your thumb is facing (that is, from left to right/top to bottom).
Step 8: It Should Look Like This!
Once it's passed through the hole, you should end up with a looped figure-eight like this.
Step 9: Tighten the Knot
Pull both ends of the twine to tighten the knot.
Step 10: Lift the End
Lift up on the end of the twine that's still attached to the roll to place tension on the loop around the roast.
Step 11: Pull Down and Tighten
Pull the end down and towards you. The loop should tighten around the roast like a noose.
Step 12: Trim Ends
Snip off the ends of the twine with a pair of scissors or a sharp knife.
Step 13: Lather, Rinse, Repeat
Repeat steps 1 through 12 at one-inch intervals until the roast is completely tied up and ready to roast.