What cross weight is
Cross weight (or diagonal weight) is the right-front plus left-rear weight as a percent of the total. At 50%, the car carries the same load on each diagonal, so it turns the same way left and right. Off by a few percent, the car can push in one direction and get loose in the other.
How corner balancing works
On height-adjustable coilovers, raising one corner's spring perch moves weight onto that corner and the diagonally opposite one. So to raise cross weight, you'd raise the right front or left rear perch (or lower the other two), re-weigh and repeat. Set ride height first, then balance with small, matched adjustments.
Front/rear and left/right can't be fixed with perches
Adjusting perches only moves weight between diagonals. To change front/rear or left/right split you have to move mass: the battery, ballast, seat position, or simply who's driving. In a left-hand-drive street car, the driver alone usually makes the left side heaviest.