They had to be publicly shared in GTSport before a certain date. Provided you'd done that, they then showed up on the livery selection screen in GT Auto in 7, you could then open them, and save them as though you'd made them in GT7. If they weren't shared by that date, you couldn't do anything about it. It also didn't work particularly well to begin with, sometimes the decals wouldn't carry over, and if you opened a livery that it couldn't retrieve the decals for, you basically ended up with a broken livery, and no opportunity to try re-importing it, as the game thought you'd already done it.
For example: This was a livery I'd made in Sport...
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All of the custom decals were mine, yet I ended up with this...
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This happened on 7 or 8 of my liveries, including my two most popular liveries (Jeff Koons Art Car on the VGT, and the Michel Vaillant Z4), which was a pain in the ass - to say the least.