depends,

u can send it to a groomer,

and they will give it a medicated bath, it will kill almost all fleas immediately,

then apply the spot-on treatment.

or u can DIY -    buy the shampoo, and make sure cat dont cakar u... trim the nails...
If u send to vet, they will just spray or use spot-on.

which will take some time to kill off the fleas.

unless the vet have grooming service.

You can buy the spot-on yourself which will be cheaper,

compare the the charge by vet. which can be RM22 more or less for one frontline spot-on treatment.

you buy one box with 3 spot-on @ discounted price is only RM57.

you wanna catch a stray cat, remove fleas and let them free again? that is pointless, because they will just get fleas again in 1 month. unless you plan to continue the treatment for that cat. easiest treatment is probably spot on. but the costs is around 40-60 depend on where you buy and what brand. or you buy those cap ayam from china brand like rm5 per tube. but maybe the cat will die, i dunno. will last 1 month.

if you gonna adopt the cat. then give them a gooood combing and a bath. then apply spot on as per basilisk mentioned. but for first time spot on maybe need those more mahal one which will also eradicate ear mites.