Children’s Outdoor How-To Book

I remember a book about a boy (possibly with his brother) and a group of friends who built all sorts of outdoor things to defend their farm & woods against a rival group of neighborhood kids.  The book alternated between the story and very detailed instructions & plans how to build the items.  

The few items I remember they built were a tree platform, a sailboat & slingshots (I think). 

The ending culminated with an apple-throwing battle and boy and his friends being victorious from all the items they had built.  

Book was paperback with black and white illustrations.  It was a rather large book and I the copy I had (lost to time) was in the mid-to-late 70’s