1. There are quite a few easy tutorials that will help you learn to design for this machine … reading through these after you have worked through the Glowforge Learn By Doing tutorials will get you well on your way.
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In addition, there is a more detailed reference on it here:
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You might want to also experiment with the Premium Features in the Glowforge app if you still have access to it…they make a lot of finicky tasks very easy.
2. Tutorials for Learning to Design Your Own Files
These are important for learning to understand the concepts of design and how to make them apply to the Glowforge software, no matter which program you use to design them.
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And if you are designing in one of the other programs:
Glowforge Interface - Correctly Save a File as an SVG for the GFUI
3. Tutorials for Working With Your Own Materials Settings
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Another source for finding Starter settings for new materials is the Beyond the Manual section of the forum. You can do a search using the name of the Material and the word “Settings” to see if anyone else has shared settings that work.
See if any of those get you started. 
(Coincidentally, to make a stamp, place a pure white image on top of a dark background. White will not be burned, black will be engraved away. Save that as a raster image and mirror any text around the Y axis before engraving, in order for it to come out correctly on the stamp.)