Look at the label of the flannel material to see if it has been preshrunk. If so, additional shrinkage will be minimal as opposed to not having been preshrunk.
Wash your flannel in the washing machine on the hottest water setting. Add a color-safe detergent and wash the flannel by itself to prevent color bleeding onto other garments.
Place the flannel into the dryer using the hottest setting. Remove the flannel promptly when drying is complete. The flannel may shrink as much as 13 percent.