Step 2: Make a Removable Panel
Leave a hole in the middle of the table. The removable panel makes it a lot easier to mount the saw and allows you to use the table with other tools. To mount a router, just make another panel and mount the router on that.
To install a panel, there are a lot of options. I had washers and smallish lag screws and was thinking about how cleaver I was, so I used them. It's a lot of work to remove the lag screws and I don't have the appropriate attachment for my drill. Instead of lag screws, you just us regular screws.
Again, make sure all of your hardware is below the surface of the table.
If you're inspired, you can even use a block plane to deal with any bumps. But the plane can only make it locally flat, so your table could still be off. But who cares---if you wanted something really precise, you'd have purchased a table saw a long time ago and you're only reading this article to see if it's a joke.