This page has some basic instructions for working with the Inkscape G-Code Tools Extension. It is not exhaustive of it's capabilities but the below should provide an effective tutorial for generating gcode for a basic engraving or plotting task.
The G-Code Tools Extension is available here: CNC-Club.ru.
There's quite a few instructions on there, but basically just get the download and put it in the extensions folder of Inkscape for your relevant operating system. Google knows this if you don't.
After that, restart Inkscape and see if it comes up in your extensions. If you can see this, you've done it right:
Then modify the canvas so you know your working space is correct. Choose mm as the units because that's what everything understands. File→Document Properties (or CTRL+SHIFT+D).
Everything you want to be drawn needs to be a path, you can convert by selecting all and Path→Object to Path.
Change your display mode to Outline to see where the paths are. Shapes will only be outlines. More advanced usage can change this behavior, but for now it's something you'll have to account for:
Then set in Extensions→GCodeTools→Tools Library select Cylinder. There are others available, but this is the most common and simplest.
This should bring up a dialogue you can edit by using the text edit tool of Inkscape. I know, it's weird, but it works. Text edit is F8 (see it off to the left?) Here I set the feed to 200 (mm/minute) and diameter to 1 (mm). Set these to the relevant sizes for your tool.
Finally select Extensions→GCodeTools→Path to Gcode and set the options here. Should be self-explanatory. Make sure you are on the Path to Gcode Tab and hit apply. This should generate a gcode file wherever you specified and draw nifty arrows on your drawing. The arrows roughly indicate the movement of G-Code generated, by you might do well to take the resulting file and view it in LinuxCNC or other CAM software that can display a representation of your job before milling/plotting.