A modern gas balloon is put up almost the same way as a hot air balloon -
the envelope is laid out on the ground, attached to the basket and fixed
to some sand bags. Then it is filled with hydrogen or helium.
When the balloon is one fourth full, the sand bags are detached and the
balloon goes up, so that the valve at the top of the balloon can close
due to the inner pressure. Then, filling continues until the balloon is
tight.
While the filling of an old, net covered gas balloon took several hours,
a modern gas balloon is ready to launch in less than an hour.