Attach the rope to the first spar with a clove hitch.

Twist the loose end from the clove hitch around the rope and then wrap the rope around the two spars as shown.  

When you start the second time around, guide the rope to the inside of the existing wraps on the first spar. 

On the second spar guide the rope to the outside of the existing wraps. 

Continue wrapping the rope around the spars following the "inside - outside" sequence. 

Once you have enough wraps to give you the strength you need for the project, guide the rope horizontally between the two spars. This is called frapping. Pull the frapping as tight as possible and then add another turn. (If lighter rope is used 4 or 5 turns may be required)

Finish the frapping by tying a half hitch. Wiggle the rope back and forth to ensure the hitch is in as tight as possible to the frapping.

Add another half hitch to finish the job!

Note: Most knotting books call for a clove hitch to finish a square lashing. By tying 2 half hitches you in fact have tied the required clove hitch.

The previous pictures showed the half hitches rather loose. In reality they must be pulled in tight to ensure the lashing does not loosen.

If you do not pay attention to how you lay the ropes you will generally end up simply going to the outside. When you do that, the frapping will only catch the outer wrap and the lashing will loosen quickly.