questions

Actually, you are looking after a temporary entry permit and not "24 hour visa". There is no such a visa on offer :)

Your itinerary is eligible for 24 hour visa free transit, but again, this is not a visa, but your right to travel through China without a visa. In reality, there is no visa at all.

You will print out your flight tickets and have those printouts with you. Also, you will declare at check-in in Hanoi, that you will travel to Barcelona via Beijing using 24 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.

Upon your arrival in Beijing, you will apply for a temporary entry permit at 24 hour visa free transit counter to be allowed to exit the airport ant to make a short exploration of the city. Anyway, you should be aware that the issuing of a temporary entry permit is always upon discretion of immigration authorities on a day of arrival. To increase your chances or better to say to make your exit practically certain, you are advised to book a simple accommodation in advance and to have a confirmation of your accommodation booking printed out. This temporary entry permit is absolutely free of charge.