(Click on the thumbnail for a larger photo)
The packaging sometimes includes an elastic mesh around the meat; remove that mesh, but don't throw it away. Let the lamb's blood drain away. Unfold the lamb meat in a baking pot (with cover); the pot can be a bit smaller than the unfolded meat since (later on), you'll refold the meat before roasting.
Peel
Cut it into slivers (a typical clove is cut into 4-5 pieces, each approx 1/8 x 1/2 x 1 inch).
Insert them into the meat, first (gently) stabbing the meat with a knive to create a hole.
Arrange the garlic slivers evenly over the full surface of the meat.
In a cup, mix the following spices:
Pour in another cup
Using paper towels, pat dry the meat's surface, and brush it with oil (all around). Sprinkle evenly with the spices; the oil acts like glue, making the spices stick to the meat. Drying the meat ensures that the oil will stick better onto the meat and thus the oil will prevent its juices from evaporating during roasting.
Fold back the meat. This can be quite tricky since the meat is oily and slippery, and you don't want to mess the even spice distribution. The easiest way to do this is with the help of a friend. One person holds the meat folded, and the other wraps the original mesh around in. If you have 4 ft of twine available, you can tie up the folded meat with it instead of using the original mesh; in fact, after the twine keeps it together, it's much easier to wrap the mesh around the meat.
If you wish, wrap the tied meat with aluminum foil, with the edges of the foil on top. This helps keep the meat moist during roasting, and it also makes it much easier to clean the pot afterwards since drippings will stick onto the foil instead of the pot. But wasting aluminum is environmentally unfriendly, so it's your choice. In the photo below, an extra layer of foil on top (for complete containment) is temporarily lifted to show the meat inside.
Bring 1 cup water to a boil, and pour into the pot. Place the tied/wrapped meat in the pot, put the cover on, and insert into the oven. The oven temperature should be 375 degrees. If you have wrapped the meat in aluminum foil, do not pour the water inside the foil. The purpose of having the water in the oven is to maintain a humid environment during cooking, to help keep the meat moist: contact with the meat is not necessary (but it's not harmful either).
Roasting time is a matter of personal preference: you may roast the meat for as little as one hour or as long as three. The longer you roast, the more crisp the outer surface becomes (and it tastes much like the original gyros), and the interior becomes slightly drier. You may use a meat thermometer for a more scientific approach: lamb is ready when the core is at 170 degrees. Make sure that you baste the meat every 40 minutes to keep it moist. If you have wrapped the meat in foil, you'll have to unwrap the aluminum foil to do that: make sure you baste it with the drippings inside the foil (which is mostly fat and olive oil), not the water outside.
When the meat's done, take it out of the oven, and let stand (unwrapped) for 15 minutes.
Remove the mesh and twine, and carve. More than likely, the meat will be so soft that it will fall apart unless you use a very sharp carving knife.
Cleaning tip: use boiling water to remove any drippings stuck to the pot.