I modified these to make them full mittens.

After knitting the thumb gusset and putting the thumb stitches on hold, I finished the cable sequence (for me, this was Rounds 8-12), did the cable sequence 2 more times as written, and then on the 3rd time through the cable sequence, I began decreasing at Round 11.

I calculated that I needed to get from 52 stitches down to 20 stitches, so decreased from Round 11 through Round 6 (8 rounds of decreases).

Working in magic loop, I had 26 stitches on each needle (front half and back half of mitt) and decreased as follows:
-First half of mitten: Sl1, K1, PSSO, work pattern to 2 stitches from the end, k2tog
-Second half of mitten: Sl1, K1, PSSO, work pattern to 2 stitches from the end, k2tog

I preserved the cable pattern as much as possible, but when my decreases ran into the cable pattern, I just purled the stitches that would have otherwise been cables.

Down to 20 stitches (10 stitches on each needle), I closed with Kitchener stitch.

For the thumbs, I decreased from 16 stitches down to 4 stitches in the same manner as above and then pulled the tail of yarn through those last four stitches (no Kitchener stitch.)

See photos for my written-out pattern modifications.
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Learning about making cables: when the pattern says to hold the cable needle ‘in front’ or ‘in back’, it means in front of or behind the working needles and it is not referencing the right side or wrong side or front or back of the work.