Don’t worry, just log into your Silverstone account and head to “Extras”. Check out your options and book parking direct from there.
The Extras section can be found either on the right of the page if you're using a PC or in the top left menu on a smartphone.
Any kind of parking (aside from Coach parking) can also be booked using the link below:
https://www.silverstone.co.uk/events/formula-1-british-grand-prix/getting-here