Put spot on wormer treatment for cats only on the back of a cat's neck, directly onto the skin, unless otherwise instructed. The back of a cat's neck is an out-of-reach position that ensures a cat cannot lick the treatment off their skin. Use all of the spot-on treatment dose to ensure maximum coverage and wormer protection. After applying, let the worming treatment dry for 48 hours and don't get the cat wet. No baths. No rain. No licking. For cat's also using spot on flea treatment, spot on wormer should be applied 2 days after flea treatment and in a slightly different spot (above or below where you apply cat flea treatment) to avoid irritation and to make the wormer work effectively as possible.
Here is a full guide on How To Put Spot On Wormer Drops On A Cat.