DSzymborski :
I think at this point, you need a basic tutorial. Something like this:
https://www.youtube.com/watch?v=rucfmsGjPow
Though ignore the part where it connects the PSU cables first to the PSU itself, assuming yours are already attached.
What are the rest of your specs? "Mystery 400W PSU off ebay" is likely to be literal garbage and depending on what it is, you can save yourself some time and effort by chucking it in the wastebin now instead of later. So you'll have to provide more information about it.
I wish I could say this was helpful but the person in the video has a different model motherboard than me, likewise he also has extra parts to connect the pins that I don't have.
I don't see why all of my specs are important seeing as I'm only having the problem with my motherboard, but whatever here's the rest:
*Intel i7 core processor
*2 Crucial 4GB 288-Pin DDR4 Ram sticks
*Western Digital Caviar 350 gb hard drive
I understand I still have more stuff to buy but that's all I have as of right now
Also I found a sticker that has the company logo for my power supply.
*Kentek 400w power supply
Hopefully it isn't junk because that would suck :/