In Heroes, Sylar kills other evolved humans and cuts open their skulls to take their powers. How does he do it?
His ability is called intuitive aptitude and basically is an analysis of complex systems, including human brain.
Intuitive aptitude is the ability to perfectly understand and utilize anything from information, theories, skills, riddles, logic, and puzzles, to the structure and operation of complex systems, codes without special education or training. The user can instantly learn and understand the complexity and exactness of organisms, objects, and even powers.
Sylar is analysing brains of people with abilities and finding out how to copy them:
Sylar himself describes intuitive aptitude as the ability to understand how things work, and how to fix them if they are broken (Once Upon a Time in Texas). However, the latter aspect of the ability has almost never been utilized by Sylar after his abandonment of his former identity as Gabriel Gray, timepiece restorer.
Sylar's ability seems to work for analyzing mechanical and biological systems.
Sylar does not need to kill someone to gain their ability. However, a physical study of the victim's brain requires direct observation necessitating the removal of the upper part of the skull, so the victim soon dies of blood loss and trauma. Victims with rapid cellular regeneration, however, can survive this by repairing the damage (The Second Coming). Sylar has demonstrated that can also "empathically" copy abilities without injuring the source, but still prefers the brain removal method out of a love of killing. He first demonstrated this by empathically copying electric manipulation from Elle.