Probably a daft question but what the hell...

If you are fitting a lintel over an opening, to support several tons
of masonry above, how do you get an adequate amount of mortar into the
(hopefully small) gap above the lintel and below the bricks it's
supporting? Do you have to just force the stuff in with a trowel and
hope you don't have any airgaps inside? Presumably if there is
insufficient set mortar in there, at best you run the risk of the wall
"settling" once the acros are removed... what's the secret?

David