Most failed first builds are decided before the first screw. A processor that does not match the board's socket, memory of the wrong generation, a graphics card longer than the case allows or a supply without the right connector cannot be fixed with care during assembly — only with a return. So the first step is a list, not a screwdriver.

The six checks that matter

• Socket. The processor and the motherboard must share one: AM5 with AM5, LGA1851 with LGA1851. The chipset then decides features, not whether it fits.
• Memory generation. A board takes DDR4 or DDR5, never both. The slots are keyed differently, so the wrong kit physically will not go in.
• Form factor. The case must list the board's size (ATX, Micro-ATX or Mini-ITX).
• Graphics card length. Compare the card's length with the case's maximum, and remember front radiators and fans take some of that room.
• Cooler clearance. Air towers have a height limit set by the side panel; liquid coolers need a radiator mount of the right size.
• Power. The supply needs headroom over the full load and the native connectors your card asks for.

This is exactly what the assembly simulator checks as you drop each part in: it measures every one of those numbers against the real specifications and refuses to boot a build that would fail on your desk. Doing it there first costs nothing. If you are still choosing parts, the order to buy them in saves you from the most common dead end, and the power supply guide covers the one part people most often under-buy.

What you need on the table

A Phillips #2 screwdriver, ideally magnetic, does almost the whole job. Add a torch, a few cable ties and the motherboard manual, open at the page with the board layout. Build on a hard table rather than carpet, touch something metal before handling parts, and keep each part in its antistatic bag until you need it.

The simulator installs parts in the order they depend on each other: case, board, processor, cooler, memory, drive. On a real desk it is easier to fit the small parts to the board first, lying flat on its box, where you can see and reach everything.

Processor

Lift the socket's retention arm and open the load plate. Line up the small triangle on the corner of the processor with the one marked on the socket, and lower it straight down with no pressure. It should drop in under its own weight. On current AMD and Intel desktop platforms the pins are in the socket, not on the chip, so do not touch the socket and do not slide the processor around once it is in. Close the plate and the arm. Closing the arm takes noticeable force, and on most boards the plastic socket cover pops off by itself as you do it.

Memory

With two modules, the manual almost always asks for the second and fourth slots counted from the processor, often labelled A2 and B2. Open the latches, line up the notch and press both ends down firmly until the latches click. Memory needs more force than most people expect; a module that is not fully seated is the most common reason a new build does not show a picture.

M.2 drive

Remove the heatsink over the slot if the board has one, insert the drive at an angle, press it flat and fix it with the screw or latch. If the heatsink has a thermal pad, peel its protective film first. Boards often have several M.2 slots and the fastest is usually the one nearest the processor; the manual says which ones share bandwidth with other ports.

Cooler

Many coolers ship with thermal paste already applied. If yours did not, a pea-sized dot in the middle of the processor is enough; the mounting pressure spreads it. Peel the plastic film off the cooler's base, a step that is easy to miss and ruins temperatures. Fit the mounting hardware for your socket and tighten the screws a little at a time in a cross pattern. With a large air tower, fit the memory first and check that the fan does not sit on top of it.

Take off both side panels and look for the standoffs, the small brass or black posts the board screws onto. Most cases have them pre-installed for ATX; check that there is one under each hole in your board and none where the board has no hole, because a stray standoff can short the back of the board.

Board, then supply, then graphics card

If your board has a separate I/O shield, press it into the back of the case before anything else, the right way up. Lower the board at an angle so its ports slide into the shield, line it up with the standoffs and screw it down without over-tightening.

Mount the power supply next. If the case has a filtered vent underneath, the supply's fan faces down to draw cool air from there; otherwise it faces up into the case. Doing it before the graphics card means you still have room to route its cables.

For the graphics card, remove the slot covers at the back of the case that line up with the top PCIe x16 slot, open the slot's latch and press the card in until it clicks, then screw the bracket to the case. Heavy cards sag over time; many come with a support bracket for exactly that reason.

Front and top fans usually come installed. If you add your own, the usual layout is intake at the front and exhaust at the back and top, which is also how the simulator's airflow view models it.

Every power cable goes from the supply to one place, and each connector only fits one way. Route them through the case's cut-outs behind the motherboard tray before plugging them in, so they do not have to be pulled through later.

• 24-pin motherboard connector, usually on the right edge of the board. It needs a firm push until the clip locks.
• EPS 8-pin processor connector, usually in the top left corner. Boards with two sockets there work with one on most builds; the manual says when the second is needed.
• Graphics card power: either several 8-pin PCIe cables or a single 12V-2x6 (12VHPWR) connector. Use separate cables from the supply rather than one cable split in two when the card draws a lot, and push a 12V-2x6 connector in until no gap is left: a partly inserted one is the most cited cause of melted connectors.
• Drives and fans: SATA power for 2.5-inch drives, and fan cables to the board's fan headers or the case's fan hub.

Use only the cables that came with your supply. Modular cables are not standard between brands, and sometimes not between models of the same brand, even when the plugs look identical.

The front panel

The power button, reset button and case LEDs connect through tiny two-pin plugs to a header, usually at the bottom right of the board. The manual has a diagram; follow it rather than the labels printed on the board, which are hard to read. Also connect the front USB and audio cables. A machine that does not react to the power button almost always has this plug on the wrong pins.

Before closing the case, connect the monitor to the graphics card, not to the motherboard's video ports, switch the supply on at the back and press the power button.

The black screen that is not a fault

A new DDR5 system trains its memory on the first boot, and that can leave the screen black for a minute or more, sometimes restarting once or twice in the process. Wait before assuming something is wrong. Most boards have small debug LEDs labelled CPU, DRAM, VGA and BOOT: the one that stays lit points at the stage that failed.

• CPU: check the EPS cable, the processor seating and the cooler's mount.
• DRAM: re-seat the memory, and try the slots the manual recommends.
• VGA: check the card's power cables and that it is fully in its slot.
• BOOT: on a new build this usually just means there is no operating system to start yet.

The simulator's power-on sequence goes through those same stages, which is why its error codes name the step that stopped the boot.

Three things to do in the BIOS

Enable the memory profile (XMP or EXPO), because memory runs at a slow default speed until you do. Check that the processor's temperature at idle is reasonable, which confirms the cooler is making contact. Update the BIOS if the board's maker lists a newer version, especially for a recent processor. Then install the operating system from a USB stick and the graphics driver from the card maker's site.

If the build is working and you want to know whether it is balanced, the bottleneck detector compares the processor with the card, and the electricity cost calculator tells you what running it will cost. The most common first-build mistakes are worth a read before you start, not after.