Building a PC is assembly, not engineering. Every connector fits one way, the parts that matter are keyed so they cannot go in backwards, and the whole thing takes an unhurried afternoon the first time. What follows is the order that causes the fewest problems, and the specific points where people get stuck.
In short
- Fit the processor, cooler and memory to the motherboard before putting the board in the case. There is far more room on a desk.
- The 24-pin and 8-pin power connectors both need a firm push. Most “it will not turn on” reports are one of these not fully seated.
- Nothing needs force. If a part resists, something is misaligned; forcing it bends pins.
- Test with the side panel off before routing cables tidily.
Before you start
Switch off at the wall and unplug the power supply before working inside a machine. Never open a power supply housing: the capacitors inside can hold a dangerous charge for a long time after it is disconnected, and there is nothing in there meant to be serviced.
What you need
A single Phillips screwdriver, ideally magnetic. That is genuinely the whole tool list for most builds.
A clear table with room to put the case on its side. A well-lit space. A phone for photographs of anything you disconnect and might need to put back.
On static: the risk is real but frequently overstated. Working on a hard surface rather than carpet, and touching the bare metal of the case before handling components, is enough for a normal room. An anti-static strap costs little if you want the certainty.
The order that works
1. Motherboard on the desk. Rest it on its own box. Everything below is easier here than reaching into a case.
2. Processor. Lift the retention arm. There is a triangle marked on one corner of the processor and a matching mark on the socket; line them up. The chip drops in under its own weight. Do not press it. Lower the arm, which takes noticeably more force than feels right, and that is normal.
3. Cooler. If it came with thermal paste applied, do not add more. If not, a blob about the size of a grain of rice in the centre is enough; it spreads under pressure. Tighten the mounting screws in a cross pattern, a few turns each, so pressure stays even.
4. Memory. Open the clips. Match the notch in the module to the ridge in the slot. Press firmly at both ends until the clips snap up by themselves. This takes more force than most people expect. If you have two modules and four slots, the manual says which pair to use, and it is usually not the two nearest the processor.
5. M.2 drive. Slides in at an angle, then lies flat and is held by a small screw or a clip. Remove the plastic film from any heatsink you fit over it.
6. Board into the case. Fit the I/O shield first if it is separate. Check the brass standoffs in the case match the mounting holes on your board, and remove any that do not, because a standoff under the board where there is no hole can short it.
7. Power supply. Fan facing a vent, which usually means downwards if the case has a filtered bottom intake.
8. Graphics card. Remove the right expansion slot covers, open the clip at the end of the PCIe slot, and press down until it clicks. Screw the bracket to the case.
9. Cables. Covered below, because this is where the problems are.
The cables people miss
24-pin motherboard power. The big one. It needs a firm push until the clip latches. A partially seated 24-pin is the most common cause of a machine that does nothing when you press the button.
8-pin CPU power. Near the top of the board, usually the top-left corner. It is easy to miss entirely because it sits in an awkward spot, and the machine will not start without it. Some boards have two; one is normally sufficient unless you are running an extremely high-power processor.
PCIe power for the graphics card. Use separate cables from the supply rather than the two connectors on a single daisy-chained cable where the card draws significant power. If your card uses a 12V-2×6 connector, push until it clicks, and check it is fully home; a partially seated connector concentrates current through fewer contacts and that is the cause behind most reported connector damage.
Front panel connectors. The small individual pins for the power button, reset, and LEDs. Fiddly, poorly labelled on some boards, and the manual diagram is essential. The power switch is the only one that has to be right for the machine to start; the LEDs simply will not light if reversed, which harms nothing.
CPU fan header. Specifically the header labelled CPU_FAN. Many boards refuse to start, or warn loudly, if nothing is connected there.
First start
Leave the side panel off. Connect a display to the graphics card if you have one fitted, not to the motherboard.
Press the power button. You are looking for fans spinning and something on screen. If you get that, go into the BIOS and enable the memory profile, called EXPO or XMP, because memory runs slower than its rating until you do.
If nothing happens at all, re-seat the 24-pin and the 8-pin CPU power first. If fans spin but there is no display, re-seat the memory and the graphics card, and check the display is connected to the card rather than the board.
Once it starts reliably, then tidy the cables and put the panels back on.
If it starts but shows nothing, our guide on a PC that powers on with no display covers the diagnosis in order. To check your parts work together before any of this, use the PC Builder.