Since aluminum is relatively soft, it can be bent to go around corners using a siding brake. Aluminum is unique in this respect, since other materials such as sheetrock will break if they are bent. Because of this property, aluminum siding can give us seamless corners that look great.
A siding brake can make neat, exact bends that fit corners precisely. In a perfect world this would happen every time, but there is a human element involved in the process and these bends usually come out a lot neater if they are planned ahead of time. Aluminum that is improperly bent must be thrown away, since any attempt to rebend it will invariably make it crack.
The secret is to create a template out of cardboard before using the siding brake. Take a large piece of cardboard and hold it up to the wall, bending it exactly as the aluminum siding will bend. When it is in place, make marks on it where the corners will be. Cardboard is a lot cheaper than aluminum, and this method allows one to get the angles exactly right without wasting any siding. Once the cardboard is marked, it can be laid against the siding and the marks can be transferred to the aluminum.
Now the siding brake can be used. There are different sizes and models, but basically the siding brake consists of a mechanism to hold the siding in place and handles for bending it at a clean angle. The siding must be placed carefully in the siding brake to ensure that the bends will be in the right places.
As easy as it sounds, this operation does require some practice. It is advisable try the siding brake a few times with scraps of siding before using it on a big piece.