Starting 1/4 inch (~6 mm) from the bottom right corner, take a horizontal stitch and emerge directly above the first warp stitch.

Flip all warp hooks to the right. This will raise every other thread to make a channel for the needle to pass through.

Slide the needle through the channel to weave the thread through the warp threads. This will be easiest done close to the warp hooks.

Finish each weft thread to the left side of the weaving by taking a small vertical stitch. Similarly to securing warp stitches, make this stitch so that the gap between each row is the same width as the thread.

The darning will look the best if these lines align with the knit rows of the garment.