only if you really want to put your rucksack on the outside.
put your rucksack back on under the poncho. put arms through arm holes (if required) in poncho.
as these ponchos are essentially a big sheet with a hole for your head and poppers up the sides i've never fathomed out why people can't put them on.
(if all could be quiet...yes...you...at the back...)
when poppered up i attack the thing from the rear, pulling the back section over my pack as i go before getting my head through the central hole and then a general pulling and tugging to get it all as i want it.