First, I highly recommend using cutting_surface.svg template that he provides with the plugin. This already has page and document settings set to the correct units for his plugin to operate. If you would like your rules and objects to be in a different unit of measurement (mm or inch), then press Ctrl-Shift-D while in inkscape to open the Document Properties dialog.
Ok! On to actually designing something. We'll start with something easy, a basic keychain. First, you'll want to select the "rectangle" tool (highlighted red, below) from the toolbox along the left side of the screen, and draw a rough shape that you would like. (Your colors and line thickness may vary from the screenshots. We will fix this in a bit)
Let's add some text, using the text tool. Just click where you want and start typing away. If the text runs out of room, you can use the selection anchors to push and pull it into the size you need.
Hey, it's starting to look like a keychain! Sweet! Ok, now we need to adjust a few settings so the laser cutter understands what we are trying to do. First, we have to adjust the Fill and Stroke of any object that we want the laser to pay attention to. On the top menu, click on Object > Fill and Stroke. It should appear in the tray on the right side of the screen, or off on its own window depending on how you have Inkscape configured. You'll have 3 tabs: Fill, Stroke, and Stroke Style. Using shift, select each object you'd like to vector cut. For vector cut lines, you will want to set it to No fill, a solid stroke(flat color), and a stroke style width of 0.001".
Now a quick little explanation of how the plugin views different objects in Inkscape.. Anything you draw in Inkscape is considered an object, and will be exported as raster data. In order to turn these into vector cut lines, you must change them to a Path instead of Object. Simple enough, with your two objects still selected click on Path > Object to Path. This will allow the plugin to export them properly, otherwise you'll just get an engraving of your cutout line, instead of actually cutting it out. Keep in mind, we only have two objects selected and converted to path right now.. not the text. We want to leave the text as an object that way it exports as a raster correctly.
Now, you should end up with something that looks like this.
The Marlin firmware has a setting inside the code that limits its max power to a certain frequency. The first section of the layer name in Inkscape defines what percentage of that max power to use. (Future reference, a 70% power level in Inkscape on my machine gives me 18ma of power, the max rating for my 40 watt CO2 laser tube. I never set it above 70 for this reason.) The highlighted layer above has a 10% power(of that max power set inside the marlin firmware), with a Pulse Per Millemeter of 40, and a feed rate of 2500mm/m. Now, we know we want to cut these two paths out from whatever material we are using, so we need to change these settings to something a bit more reasonable. On my machine, I know I can cut 1/8th inch acrylic with a power level of 68%, PPM of 40, and Feedrate of 250mm/m. So, my layer name will be: 68 [ppm=40,feed=250]
Select both paths you would like cut out at the above settings, right click on them, and move them to that layer. Next, we will move the text to a separate layer and adjust its power settings to 15% power, with a feed rate of 3000mm/m and PPM of 40. If you have empty layers, you can remove them with the blue minus button in the layer tray. They don't hurt anything, but it makes it easier to keep track of which layers/power levels you are using.
For this tutorial, I'll be using an SD card to transfer the gcode to the laser. Click on the Advanced tab, and make sure "Are you using Pronterface?" is *not* checked. Leave the rest of the options as default, and click on Apply. Once it's done processing you'll get the below notification. Check it carefully to make sure it looks like what you are expecting.
Keep in mind, this is just a very simple tutorial to get your feet wet. I'll be writing another tutorial soon that involves importing artwork and going over how to organize your layers and objects to cut in an order that makes sense. (inside cuts first, before outside, etc). Have fun, and don't burn the house down! :)