The Sheet Bend, for joining two ropes of un-equal thickness. The bight being formed by the thicker of the two ropes. My example uses two ropes of equal thickness for simplicity.
Start by forming the bight
With the bight formed, pass the working end through the bight towards the side of the bight with the end, round the back of the bight, then underneath the standing part of your rope from the right, but over the top of the bight.
It is important the two ends leave the knot on the same side. Otherwise the strength of the knot is reduced.