Are you having trouble doing this? If so, what? Tell us what's happening.
If you haven't done anythig yet, then here is what you do:
First, make sure you are on manual, not automatic (iTunes>Preferneces>Devices--check box for "Prevent" iTunes from syncing automatically with devices).
You plug in you iPod and it should appear there on the left side. Then you go under music (or a playlist), click on the song (or songs, or playlist), hold and drag it till it covers the iPod. The iPod will "shade" and when you release the song, it will be copied onto the iPod.
It all depends on what your settings are. For example, you can do a 3-finger drag--but it has to be checked off under your trackpad preferences. However, here's a way to do it no matter what you've got set up: Point at song with arrow. Click down the trackpad with one finger and hold it down so that the song is picked. Use another finger from the other hand to drag the song to iPod. Release.
Now if you've set up your trackpad to do so, you can do this with one finger and a double-tap or a three-finger drag. Check out:
This is where you can not only see all the gestures you can do on your trackpad and how they work, but check off which ones you want and which ones you'd rather do without.