Filing (in the DIY sense) seems pretty simple: you grab a file, then you just scrape it across the surface until the object you're filing is as smooth as you want. However, Make points out that there are a couple of different methods to choose from.
You have two basic methods: straight filing and draw filing. Here's the difference between the two:
The first and most basic filing technique is called straight-filing, and involves pushing the file lengthwise across the work, making contact only on the forward stroke.
Straight-filing is typically used to rapidly remove material.
If a smooth surface is what you're aiming for, then it's best to use the draw-filing technique.
In draw-filing, the file is held on both ends and is pulled and pushed across the material crosswise.
Pick the right one, and you'll get the results you want. Head over to Make for more info.