Building a custom PC involves defining its primary use (e.g., gaming, productivity) and setting a budget. Key components include a CPU (Intel Core or AMD Ryzen), GPU (NVIDIA GeForce or AMD Radeon), compatible motherboard, 16GB RAM (or more for heavy tasks), SSD for speed and HDD for capacity, a sufficient wattage PSU with an 80 Plus rating, a case that fits all components with good airflow, and necessary cooling solutions.