If you have a measuring tape (preferably clothing measuring tape that is able to fold easily around body) wrap around the body until the measuring tape overlaps. Don't pull too tight. It should be naturally sitting at following locations to take measurement. If you don't have clothing measuring tape, you can cut a thread or ribbon that matches the below.....
Bust:wrap around your body from the center of the breast, the largest part around the breast until the tape meets
Waist:wrap tape around your body, right above the hip bone, typically right at the belly button
Shoulder: place tape from edge of left shoulder to edge of right shoulder
Length:to estimate where the length of the robe will land on you, lay tape from high point shoulder (typically edge of shoulder) down, over your chest all the way down to specified measurement on the chart
Sleeve Length:to estimate where sleeves will land on your body, take measuring tape from high point shoulder (edge of shoulder) down to end of hand