They suggest you do it after being sedentary for long periods (like sleeping, getting off a plane, etc), as it is those times that the lymph system most needs stimulating. But really I would say whenever it can be easiest folded into your routine.
Also - the instructions someone gave have a common 'mistake'. I dont know which is technically right so I put mistake in quotes...but I have been specifically told NOT to start at the extremities.
You should start by activating the big nodes, ala the left armpit (which processes 75% of your entire lymphatic drainage), then breast/tummy area, then right over the leg/hip joint, then behind the knees and THEN from the extremities toward the heart.