Copy and Compose (1969) by Weathers and Winchester, was written by two English teachers from the University of Tulsa. I bought it idly in a used bookstore near the Univ. of Oklahoma in 1976. I remember it hadn't yet been shuffled to a shelf and was in a box with another 30 or 40 copies of the same volume. It was a slim volume for only a buck so I bought it. Over the next few months I became entranced with it.
The book's format for learning sentence structures is to provide significant examples of each (loose, periodic, inverted, compound, etc.), and then instruct the student to copy it exactly, followed by producing multiple examples of the *same structure* now filled with the student's variations in content. This I did, and repeatedly. I made significant notes, and reduced the explanatory segment for each sentence and put them on index cards. In the mid-80s these migrated to the computer. Then a database which I've maintained through many operating systems over 40 years. I've given my notes/files away so many times, I'm delighted to find that if you google "copy and compose", you'll quickly find someone's copy of my handiwork, now cleaned and buffed, but essentially the same thing I produced in 1976.
Over the many years since then, I have repeatedly come back to this thing and produced another packet of pages emulating the structures. I would periodically get my writing moving forward by selecting 2 or 3 structures at random and seeing where it led me. I will be continuing this all the days of my life.
I have proselytized endlessly for this approach because, as a musician, I am a great believer in knowing your scales and arpeggios so well that you construct music with them, and without directly perceiving your expressions as a reassembly of modular constructs. Again, my endless recitation of the Pablo Casals's quote, then in his 90’s, "Scales and arpeggios--every day!"
Sadly, "Copy and Compose" is long out of print and my old copy is now 30 years gone. Fortunately, Weathers and Winchester didn't stop here. They later produced a book called Strategy of Style, and even later updated it as "The New Strategy of Style." These two include all the material presented in "Copy and Compose". Good luck finding them.