Thread an embroidery needle with a length of embroidery thread no longer than 32 inches (~81 cm). Don't knot the thread.

The eye size of the embroidery needle should be large enough to accommodate the yarn, but not much larger.

Make a horizontal stitch about 1/4 inch (~6 mm) long with the needle emerging at the bottom right corner of the drawn square. Pull the thread through until there's a short tail, about 1 inch (~25 mm) long.

Wrap the thread around the far right warp hook.

Pull the thread taught. Maintain even tension, but don't pull tight enough that the fabric pulls or the hole begins to close.

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