Each of the images below is a link to a step-by-step geogebra applet through which it is possible to move forwards and backwards in the construction, or watch how the construction is solved in a short animation. The blue “circular” points in some applets are movable.
These applets are gathered in this this geogebra book.
regular octagon, given the large diagonal
regular octagon, given the edge lenght #01
regular octagon, given the edge lenght #02