I usually make Scooter and Skip pyjama pants for Christmas or birthday.
I've used this pattern for Scooter several times.
I always pre-wash and machine-dry flannelette because I want the final sewn size to be right. This pattern calls for elastic across the back and twill tape ties attached to each side of the elastic. I also anchor the elastic by stitching through it and the waistband down the centre back seam. I usually try to match the horizontal lines but this fabric only had vertical lines so I didn't have to do that.
I liked the Thunderbird motif and again, didn't need to worry about matching horizontal lines. Skip also got a nifty label for the centre back.