Over at Ponoko, we often make parts that are used as stencils. PETG is a popular material for this purpose as it's thin and transparent so easy to align and use. When creating your design remember to think about what needs to fit in the stencil. For instance, if it's to be used with a pen, you'll want to make sure your holes are large enough for the tip of a pen.
Because you need internal parts to stay attached for stencils to work some additional design steps are needed when creating stencils as opposed to simple cut parts.
You'll want to:
1. Add a rectangle around your design, to ensure you have a suitable amount of material around the stencil that has smooth uniform edges
2. Add bridges into the design to keep the internal parts attached.
Here is an example of a design that is not ready to be a stencil vs one that is:
Ways to achieve bridges.
1. Use a stencil font
If you are using text you may be able to find a 'stencil' font that has done the hard work for you. Just remember to expand the font to vectors before exporting your file.
2. In Adobe Illustrator design software - use the Pathfinder tools
Accessible via 'Window > Pathfinder' these tools make quick work of tasks that could be otherwise extremely fiddly and time consuming.
Here the 'minus front' tool can be utilized to remove one object from another.
You can draw some small rectangles over your existing shapes and then minus them back to achieve bridges.
Check your chosen material page for the minimum feature size to determine your bridge size (aka the size of the rectangle to used in this example). Typically the smallest you want to go is 1mm.
3. In Inkscape design software - Use the difference tool
This follows the same logic as the Illustrator tools above. You can remove rectangles from your parts using 'Path > difference'
Many design softwares will have a tool similar to the ones shown above. Googling 'XXXXX software minus tool' often produces helpful content.