Where invites show up: in your creator panel, with a notification. The offer shows the campaign, the content plan with the products, and the posting dates. For Paid Collab it also shows your fee.
The clock: you have 24 hours to answer. If you do not answer, the offer expires and the spot opens for another creator.
Before you accept: read the content plan and the dates. Only accept if you can post on the brand's dates.
Accepting means you commit to the plan. Read the box "What you are agreeing to", then press Accept.
After you accept: for X-Deal, the brand ships the product and you will see a notice when it is sent. For Paid Collab, the brand pays you directly and uploads proof, and you confirm the payment before you can post.
Saying no is fine now and then. Declining early is better than a no-show, because a no-show earns a penalty.