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Find and install iFile from Cydia (free, although you get pop-ups urging you to buy it)
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Find a valid .deb (Debian) package (for my projects, go to the Releases tab and hit the big green button)
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Tap to open the .deb file (might launch into separate app, or have an Open In... prompt)
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Tap "Open With...", and select iFile (normally in the upper right-hand corner)
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Once in iFile, tap Installer from the selection sheet that pops up
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After the text stops scrolling on the black Installer screen, respring your device
Enjoy!