Why don't you read the instructions on the label?
There are variations in application requirements from product to product. Generally, a pre-dampend carpet will absorb the protector better. If appropriate, a just cleaned carpet should be acid rinsed as protectors are not compatible with most detergents.
When it comes to new carpets, many think that the pre-treatment with an acid rinse to to acidify the yarn. This is partly true as a new carpet will have, perhaps, contaminants present which will resist the penetration of the protector. These can include spinning oil and moth proofer, so an acid rinse is, IMO, beneficial if appropriate to the brand of protector used. Another factor is that acid rinses contain surfactants which aid penetration of the fibre, in other words, they're easier to wet.
As far as I'm aware, all water based protectors should be brushed in and the pile groomed afterwards, skirting boards, doors etc wiped down. Do NOT turbo dry a protected carpet in the early stages. This may dry the carpet too quickly before full penetration has occured.
As for the curing aspect mentioned by Mark, think of a protector as being like paint. After a few hours it will feel dry, but it's still very easy to rub paint off with your fingers for upto the next 24 hours or so. Ditto for protectors. Curing time varies from brand to brand. It typically varies between 24 and 48 hours.
Safe and happy protecting
Ken