When you have all the parts, you need to assemble them, into a working machine. Take it one step at a time.
Before you can load Linux , you must assemble the hardware. And this is Issue-One, all about doing that. Remember to write down the SERIAL numbers of every hardware item.
DON'T PANIC! It's easy - really. Like MFI-DIY-Furniture; its all a bit confusing, but there are EXACTLY the correct number of screws and cables and sockets and things (don't drop them). You need two tools - a Phillips cross-head screwdriver, and PATIENCE.
Every circuit board has a number of 'jumper' switches with strange names, to be set correctly. Multi-circuit cards (eg ide + IO + floppy) should have 'disable floppy circuitry' jumpers (you already have one installed on another card).
Most modern PCB's have all jumpers clearly labeled, so that you could re-configure them in 5 years time when you've lost the instruction sheets (many don't).
All connectors have a 'PIN-ONE' indicator in one corner.
Match pin-1 on the card, the cable and drive. Most ribbon cables have an ink line following wire-one. Occasionally ribbons are put upside-down onto the connectors - but this is rare. Think it out. Some connectors will only fit-one way up (look before losing your head in the tower).
Many connector-plugs are 'tag keyed' so that they only fit one way into their sockets. Make sure that this agrees with the pin-1 layout, and decide how the cable will twist as it goes from device to card.
Power connectors have two plastic corner cut away, but it can still be confusing, as they are a tight fit, and need a bit of push, even when thay are the right way round.
MANY connectors require a bit of FORCE to push together and pull apart.
Cards do NOT push smoothly into the system board, you have to wiggle them so that you believe they are seated correctly, then push one corner first, and PUSH.
During this unnerving activity, the system board will bend, so YOU pincer it (you not the case), or else the adjacent cards will slide out (because the system board pulled away). I didn't notice that, as it looked OK.
Beware of solder on the main PCB, it will cut you (also clean your hands first).
Be aware of STATIC electricity. Don't wear ANYTHING that is 10% non-natural. Take your shoes off. Touch around you, touch the radiator, touch the system case, touch the EARTH pins of the PCB (eg speaker out pins). Never touch or get grease on the edge connectors (finger sweat contains all sorts of chemicals, that eats into the copper connectors). Keep the cat out of the room.
Get the shop to seat the CPU and the memory in the right bank, the right way up.
Before replacing the back-panel (with system board) into the chassis, check that you can easily locate all the sockets and pins for the various connectors. It isn't so obvious later. You can scribble markings on a photocopy of the manuals board map.
Then with the back-panel installed, connect up the (two side by side) plastic power connectors to the board, along with the LED wires from the front panel, the speaker (etc).
Allowing for a plethora of different component sizes, every screw in the machine matches up! There is a confusing excess of bolt-holes, and there are two different threads (disk-drive chassis bolt, ISA-card bolt) as well as a range of depths (don't drill through a hard disk).
Except curiously the plastic pillars that are supposed to hold the motherboard. It tried my patience, I didn't believe it. I thought about it again, left it until tomorrow, and still don't like it.
Panel sections where optional CD-ROMS poke thru, and those metal covers where the cards go, have to be broken to be removed (unless you are into metal fatigue). Do not worry, this is normal.