It's pretty simple to do. Here's a website directly from Intel on how to install a new Pentium 4 CPU:
Browse down to about the middle of the page or maybe just a little bit more below that point and you'll see some very detailed explanations including pictures on installing a CPU. By reading that, it should make it easier. When you do buy a CPU retail, it will come with a huge poster showing you step by step instructions also. So colorful.
It's just what you said. Put it on, fasten it, and ready to go. CPUs use zero insertion force so it should only go in one way and plumps into the socket smoothly. You pull down the lever after that and the CPU is on!!! On the bottom of the heatsink, there are normally a thin layer of thermal compound so when it makes contact with the CPU, it helps to transfer some of that heat out. Now for the heatsink/fan. This can get a little tricky. There are retention clips from the heatsink that need to lineup with the bracket so it will have a snug fit when you lock it up.