They also have an adjustable waistband, just like this pair. And I thought I share how I did it. So say hello to my very first tutorial (but please note: this might not be the best way to do it, it's just the best way I figured out to do it. My approach to sewing is very much trial and error, and this is what I came up with. And please keep in mind that English is not my first language, if something reads funny to you...)!
First, follow the directions in the book until you have two pairs of trousers: the outer one and the lining pair.
Take the outer pair and fold the raw, unfinished upper edge about 1/4 inch toward the wrong side. Press. Repeat with other pair.
Next, take the LINING pair and measure about 3 inches on each side from the middle seam. Mark each side and sew a buttonhole.
The next step is to fit both pairs together, with the wrong sides facing. Pin the upper edges together and stitch around. Remove the pins along the way.
Now, stitch around the waist once more, this time about an inch away from the edge (that measurement depends on how big your buttonholes are, the buttonholes should be in between the two seams. The end result should be a tunnel on the upper edge of the pants:
And the last step: sew on buttons and insert buttonhole elastic.
All you need to do now is finish the trousers by topstitching around the bottoms of the trouserlegs. And then, they are ready for some playground action!