The buyer should click on the "Hire" button on her give up.
That will deliver you a message (via email or Upwork message) and also change the repute of the venture on this web page.
Upwork -> Find Work -> Proposals
You want to appear a few factors indexed in the "Offers" segment.
When an activity is there, you may "Accept" the offer.
That's it. I explained in this video please watch it.
If you have any doubts, Please let me know.