Regret my delay in answering your question. I believe the two previous answers are sufficient

You can go on their website http://www.pinellascounty.org/park/camping.htm create a login. Their site stinks it's not easy to navigate. OR you can call 3500 Pinellas Bayway South, Tierra Verde, FL 33715 Information Line: (727) 582-2100

This campground is very popular and hard to get reservations for. BUT... We managed to reserve two days and once we arrived discovered cancellations which allowed us two more days (we had to move to a different site, but it was worth the trouble). Please note that there are two different loops, one for small rigs and one for long, tall rigs. There are a lot of very low tree branches in the spaces on the 'small rig' side, so pay close attention to which spaces you choose!

It is a park owned by Pinellas County so you have to set up an account on Pinellas County website to reserve

Check out the QandA from 5-31-21 below. It's got lots of useful info: Any tips or tricks as to how to score a site here? Many have shared their frustration with the reservation site...

Use the info at this website. https://www.pinellascounty.org/park/camping.htm#fortd

You have to use the Pinellas County's website which has a page for Fort DeSoto reservations. It's very important to note that Pinellas County residents are able to make reservations 7 months in advance whereas non-residents can begin making reservations 6 months out. Therefore, it can be very difficult for non-residents to get reservations during peak times.