Secret second hobbies

The one you see is about 48" across. There is another one behind that one, and two more that are about 38". I have no clue about surfacing, I was guessing maybe set some kind of a rail system to ensure flatness, and go after it with a sander or router? I'm still about six months away from these being dry enough to work on, but getting my ducks in a row will help when the time comes.
Build a router sled that will allow you to run a router back and forth over the top with a wide bit.