I get the impression you have no electronic, or even mechanical assembly experience, and you're playing with dangerous, and deadly voltages.
110v isn't deadly. Splicing a switch into the positive wire shouldn't be too hard. You need to use some thick wire, but you can steal that from an old power cable.
As long as you are careful and double check everything and don't do anything stupid like leaving exposed wired. I'd use a multi meter to check for continuity and shorts before even thinking of plugging it into the mains.
Sure it's possible to mess it up, which is why you position the computer far away enough away from the power socket that when you turn the power on at the wall that if it blows up that you're not going to get injured. First turn the power on when the switch is off, then turn the power off, unplug it and turn the computers switch to on, plug back in, turn the power on & see what happens.
If it doesn't work then you'll need to start thinking about checking voltages while it's turned on, but you can unplug it all and become calm before you try that.