How does the interaction and transaction go when making a brand deal?

Hi! As you might notice from the title I'm a newbie at this making money online thing. Not sure if the right forum but here goes:

Brands have been starting to mail me about sponsorships/shoutouts etc.
I want to know how you guys do it. Do you ask them to first send the money and then after you post a shoutout on instagram? Or do you post the shoutout on instagram first then you get the money?

Because I'm worried about scams, what if they don't pay me and I believe they might think I scam…

How does the interaction and transaction go when making a brand deal?