In this method of laying veneer we see the utilization of the gelling characteristic of hide glue. Equally, we see the ability of the glue to override our determined misuse of it by first roughing the surface of the substrate with a toothing plane in the misguided belief that the glue works by mechanical rather than specific adhesion.
It's also worth remembering that the earliest veneers were sawn. Subsequently, when veneer slicing replaced sawing, there were 26 pieces/inch compared to today's average of 40. In other words, all veneers are not created equal and modern ones will more readily fall apart if subjected to hammer veneering.
Of the small collection of tools needed to veneer using hide glue, the veneer hammer is vital. It was shop-made, like the one shown here composed of a piece of dowel and a piece of MDF fitted with a round-edged 1/8" thick brass strip. By pressing the strip hard onto the veneer, your weight is concentrated onto that small contact surface.
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There are various preparatory techniques for sizing the substrate but the principal steps are as follows: Cover the substrate by wetting with hot glue. Lay down the veneer, then squeeze the glue line level and thin by pressing it with the veneer hammer. As the glue goes through its gel to solid phase, continue applying pressure.
It's a time/heat/pressure relationship, with the pressure being applied by the narrow bar of brass, which is continuously pressed back and forth over the veneer. If you are unsuccessful, as indicated by the veneer lifting, the glue can be reactivated with the aid of a wet cloth and heat from an iron.
Hammer veneering can certainly be learned, and if you intend to make or repair small parts, it's a good use of time and energy. If you are only passing curious, try it by all means just to realize that this is a skill that requires serious time and practice to do what otherwise could be done in a press.
Ian Kirby talks more about hide glue -- its history and its uses -- in the December 2007 print Woodworker's Journal.