You can do this by using a Trisquel's net-install ISO. Those ISO files are specifically designed so you can either burn them to CD, or write them to an USB stick.
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To write an net-install ISO to an USB stick, you can run the following commands:
"/dev/sd#" is the device where you will write the image to.
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You must replace "#" with the appropriate small Latin letter ("a", "b", "c", ...), so the name corresponds to your USB stick.
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For example, the name could be "/dev/sdc". Be careful to not mistake your USB stick with another device, because the commands above will delete everything on that device.
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You can see which device your USB stick is in Disk Utility.
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The "sync" command makes sure everything is written on the USB stick, so you don't remove it prematurely.