-Normally I would say close the case and then hook up the peripherals but this is the real world. Unless you're perfect you have screwed up somewhere that will make it necessary to access inside of computer later. No offense of course, but it happens and its usually small stuff that gets you.
-Your mouse will plug into the back of the computer along with everything else. You will notice small ports in the back where they fit in and noted by the I/O shield. Simply plug the connector into the USB port or PS/2 (usually green).
-The keyboard is like the mouse and usually hooks up next to mouse. (usually blue) Plug in snug and that's it.
-The printer uses the parallel port that is noted on I/O shield. Plug in and secure with the small screws on side of plug.
-You may want to hook your speakers and microphone into sound card. This is also noted on the back of sound card for the plug in's.
-Hook up monitor and make sure that connector seats well. Be sure not to jar it to much, you can unseat the Video Card if it has a bad seat or not much of one.
Go ahead and plug power connector into wall and then into the back of the power supply.