Usually just sitting still and repeatedly pumping the footbrake pedal firmly will do the trick. The pedal should lose some of its travel as the adjusters take effect.

Definately slacken off the handbrake cable adjustment first though. If it's overtightened it can prevent the adjusters doing their stuff.

The adjusters may have rusted solid - if normal ones they will be a screw rotating through a nut and the unused section - due to worn shoes- may have rusted and now be seized.

Strip down and check they move freely - on each wheel..