You can assemble the tetrahedron several ways.
It really does not matter. Eventually the structure should look like
the following diagram.
My favourite method
though, is to start with the spars laid out on the floor as shown in
the next diagram.
Tie all the lashings,
and then lift the centre (at the tripod lashing) and swing #1 to #2,
#3 to #4, and #5 to #6. It may seem complicated the first time you
do it but it is a very quick way to assemble the tetrahedron!
Secure the spars at the
corners with 3 more square lashings.
The final touch is to
add the thrower that will launch the tennis balls. It will look more
or less like this!
The thrower itself is
made from a shallow tin can like a tuna fish tin. I attach the 1m
(3ft) bungee cords to it with filament tape (used for packing).
Remove the hooks from the bungee cords at the tin can end to prevent
catching people's hands! The hooks can be left on the other ends
to help you attach it to the corners of the tetrahedron.