Decreasing in filet crochet is incredibly simple. You either simply make the row shorter or start the row with slip stitches, and this works whether you’re decreasing on top of solid blocks or spaces.
To decrease at the beginning of the row, simply slip stitch in the number of stitches necessary. In a 4-dc mesh, you would be making a multiple of 3 + 1 sl sts. The last stitch you sl st in should be a dc (one of the outer stitches of a block). It will also be the st you make your turning chain in (counts as 1 dc)

7 slip stitches were used to decrease by 2 blocks.
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