With the rectangular frame assembled, begin attaching the joists, 16 inches on center. Support each joist temporarily with whatever you have handy (spare concrete blocks, scrap wood, etc.) and fasten it to the frame using two screws. Continue to check that your work is square, using a carpenter's square (picture).
When the internal joists are in place, go back and add a second joist on each end, to form a "double header," giving you extra support at the ends.
Note that, at this point in your floating deck construction, there's no need to restrict the deck frame to what will be, eventually, it's proper location on the concrete deck blocks. The frame-and-joists structure you're assembling will be light enough to enable you to rearrange it on the deck blocks later, at your leisure. At that point, you'll start checking for levelness, something you needn't worry about yet. Right now, you should be concerned with getting everything square (photo).
This freedom to move the frame as needed while you work comes in especially handy if your deck is located in a tight spot. For instance, in my sample project, it would have been difficult to drive screws into the side of the frame that will abut the privacy fence, because there's not much room to work in there. So it was fortunate that we didn't have to.
But in Step 5, it will be time to place the frame-and-joist structure into their final position on the concrete deck blocks. It's at this stage that the floating deck really seems to take shape....