Start with a single Timber Hitch around both poles. Wrap three or four turns around the two poles in one axis followed by three or four turns in the other axis. Tighten the lashing by surrounding it with three or four frapping turns. Finish with a Clove Hitch.
Use: The Diagonal Lashing (ABOK # 2115, p 343.) is used to lash two spars together. Unlike the Square lashing which works for right angle crossings, the diagonal lashing secures poles crossing each other at a variety of angles.
Tying it: An initial Timber Hitch surrounds both poles. The choice of a timber hitch is important. Sometimes there is a gap between the poles. Pulling on the Timber Hitch closes the gap and allows the lashing to proceed with poles touching. A clove hitch around one pole could not be used to pull the poles together and might come untied.
Use: As shown here, the diagonal lashing is used to join two diagonal poles that are being used to brace a rectangular frame. The location of one diagonal in front and one behind explains the gap between the poles commonly found in the center.
Options: The animation shows the lashing made on poles that happen to be at right angles. Under such circumstances a Square Lashing might be as effective. However, when the angle between the poles is closer to 45 degrees, the diagonal lashing is more appropriate.
Frapping Turns: The turns surrounding the lashing at right angles exert a tightening effect on the lashing. These turns are known as Frapping Turns. Pulling them as tight as possible makes the Lashing more secure. Various techniques are recommended, but I'm indebted to Dana Holgate for the following: wrap the rope around a stick, stand on the pole, bend your knees, hold the stick across your thighs and then pull by straightening your legs.