Stage 1, first catch your cat. This may involve shaking munchy biscuit box, wafting open Hi Life pouches around.......to no avail.
Stage 2, long apologetic phone call to said Vet Receptionist explaining that that you have seen neither hide nor hair of said cat since you went to bed last night, with kitty snugged up in cat bed! The kitty that miraculously re-appearers as soon as Vet appt has gone.
Said cat, if caged in a bedroom, ignores litter box and uses bed instead
2 new mattresses are quite enough!