Shavehorses have been built in many different variants, but the working principle is the same: allowing efficient use of draw knives, locking the wood in position while working on it.
Here is a video by Ben Orford explaining its use
I built my shave horse out of scrap wood. Basically I used a large old board, and some smaller 8cm x 5cm I had around.
I wanted to have something very stable and heavy, that would take a lot of load. I decided to build the three legs of board sections. I designed the joinery to be rigid and reduce any movement.
I built the moving part creating a solid and square wood frame.
I drilled holes in the vertical bars.
I then planted in the horizontal board large metal screws with washers to allow the vertical structure to pivot.
I used a simple hinge to fix the lower end of the sloping board to the main bench.
A simple free moving wood block allows to change the slope, adjusting the clamping on the work piece.
The following videos were interesting and useful for me during my research, from experienced craftmen. I recommend the videos by mr. Harry Rogers.