If you know that the snake is really poisonous and could quickly kill the person if the poison spreads, constrict the extremity so that blood flow is reduced. Note that this will damage the bitten area. Otherwise, if you know that it's slow and a safer poison, remove constricting items so that the local area will not be damaged too quickly (and will not need to be amputated).

The affected person should move as little as possible and try not to use the poisoned area. This way, the venom will spread more slowly and the affected area will be harmed less.

Take the person quickly to a place where emergency treatment can be provided (hospitals, clinics, specialised snake treatment locations).

I found an article with much more information that could help you:

https://www.emedicinehealth.com/snakebite/article_em.htm#first_aid_for_a_snakebite