Apply contact glue to the lower inside edge of the hat sides. Then wait a minimum of 5 minutes.
You may find the fabric has soaked all the glue up. If so it’s a good idea to apply a second layer of contact glue to both tabs and inside the lower edge of the hat sides. You want this joint to be strong.
Wait after you apply the second glue layer, it should not be gooey but almost feel dry with a very slight tackiness.
Note contact adhesive is not easily re-adjustable, so mark your hat centre front point and centre back on the brim and the sides with tailor’s chalk/visible textile pen, or use a pin.
Make sure these marks align. Then begin with the centre front tab.
Press to the inside of the hat. So that the brim is firmly pushed up to meet the hat sides.
Repeat for the centre back tab. Then repeat at the side quarter mark, then the tabs in between.
This pattern is made to fit exactly. If yours does not fit then it has not been cut exactly to size or your covering fabric is too thick.