Give them options
A useful starting point is one tray per cat plus an extra, in separate locations. Keep trays away from food and water, busy passageways and noisy appliances. Choose a tray large enough for turning and digging.
Keep access comfortable
An older cat may need a lower entrance. Try the litter they already know before making changes. Scoop regularly and keep the tray clean without strong fragrances.
Treat a change as information
Toileting outside the tray is not spite. Note what changed and speak to your vet, since health and stress can affect toileting. Straining to pass urine or producing little or no urine needs urgent veterinary attention. Never punish a cat for an accident.