How do I create a radius/chamfer onto my part using a file?

In general: To create a chamfer, angle the file so it’s tangent to the edge of the piece and routinely push the file away from yourself.

On the lathe: Turn on the lathe and position the file so that it will cut in the sharp direction of the teeth. Be extra careful when filing on the lathe! Things like long sleeves or files without handles can quickly become safety hazards. Here’s a video of how to safely file parts on the lathe.