I've assembled computers/PCs and large rack systems for 30-some years. All the equipment I run today are self-built. In that time I've done my bit of damage to parts - but I've NEVER had to solder/replace components already soldered to the motherboard. PSUs on the other hand go bad _often_ - so I have a simple PSU tester I always use to make sure all the +3.3v, +5v, +12v, GND and other wires (we used to have a -5V too) are correct from the PSU. Motherboards/systems will either go dark or behave VERY strangely if the PSU is out of whack. Heck - at times I just replace the PSU to be sure if I know it's getting old. This is why I have a few PSUs sitting unconnected too - these suckers _will_ go bad eventually, in particular if you like me run the systems 24/7.
If this is the first time you're assembling your computer yourself (welcome to the club!), be aware that even small mistakes can be costly. Depending on your CPU and socket brand, just a slight out of balance or worse direction of the CPU in the socket can bend leads resulting potentially in loss of CPU and/or motherboard. Close inspection of the leads in the socket (you may need a microscope) to detect if any are bend can be needed (it is possible to fix that but it requires a steady hand, micro-scope and some small utilities to access individual connectors). If you can, test the CPU in a different motherboard to isolate the issue. Another thing that can go wrong is _not_ grounding yourself as you handle the components like motherboard, CPU, RAM etc. - some homes have a lot of electrostatic "stuff" around, dry interior etc. makes sparks fly simply by walking. This is DEADLY to a component containing billions of transistors. And don't use those that aren't attached to a wire just in case they still sell those on Ebay. Once exposed a component/board may be totally useless or at least it can be hard/difficult to identity the components damaged let alone find replacements for them.
When you first get started on an unknown board/cpu - start out simple. Don't plug any cards in, use the internal VGA (most boards today provides some kind of video out) and be sure you have the board status connected to a speaker and some boards have 7segment displays or some color LEDs that indicate POST status. With _just_ the CPU in the socket and connected properly, you should receive beeps on boot that RAM is missing - but that's a good indication that everything is so-far working. Then you insert as little RAM as possible (pay close attention to your manual - some CPUs require the memory to be banked in pairs) and when you do POST now you can now access the BIOS. This will allow you to run hardware tests, validate connections of fans and other external connections you may need/want to your case. Still, no external PCI cards at this point. With a successful BIOS that sees fans etc. insert/connect HDDs/SSDs and all the USB lines - and yet again, a POST will allow you to test that the devices are seen - plug in a boot USB to be sure the port is seen and can be used. At this time, I would add any PCI card I have, again verify that the BIOS can see them, and then close the case and do the OS install.
Mounting your motherboard in the computer should these days be less troublesome but with all electronics, you have to take care not to damage it. Be sure it's grounded properly to the case and your offset holders are at the correct height (if not, you can crack the motherboard PCB and trust me, those errors are REALLY hard to find!). With new cases they provide you with the offsets - if you have to purchase them it's really important you get the right hight for the case you have. And all of the offsets must match the mounting holes in your motherboard. Inserting RAM should then be relatively simple - but I have had motherboards with little support in the RAM area, which means you should only insert RAM before you put the motherboard into the case - having the motherboard bend/buckle as you push the modules into their sockets isn't fun considering some of these suckers are expensive.
Not seating memory right usually appears in the BIOS as missing RAM or mis-identified. If the RAM is "old" it may have some corrosion on it, which you need to remove (rubbing alcohol is an excellent way to do this). But you can determine if the CPU works by simply powering up with just the CPU. Note - CPUs today typically require their own 6->8 pin power connector.
The secret to my success is breaking down a problem till you isolate to a very narrow scope of areas things could be bad in - or just have one potential area. I start with the PSU always - and work from there. Good luck.