Step One
Lay out where your new loft will go using four pillar blocks or four concrete blocks. You can stack additional blocks on after to raise loft height above the ground but for now just start with four. Measure from opposite corner to opposite corner to get your blocks square. The blocks are square when the two distances are the same. Now using a 4 foot level and an 8 foot straight edge level all four blocks. Once your base blocks are square and level you can add additional blocks if wanted to raise the loft height above the ground.
Build your floor joists using 2X4s on 24" centers, using 16 penny framing nails or 4" sheet rock screws. Make your floor a full 8 foot long but only 46" wide, other wise the finished loft will exceed 48" and might not fit into the back of a pick up. Attach your floor, once again 46" by 96". I use 3/4 OSB tong and groove sub flooring but 3/4 plywood can also be used. Use 2" sheet rock screws every 6" or you can use flooring nails but just make sure they are sunk below the flooring so your scraper will not catch on them.
IMPORTANT
Use the Concrete pier blocks with the metal anchors so you can bolt the loft to the concrete blocks.
Also, I would recommend using 6 blocks instead of 4.
There has been two lofts that I know of blown over in hurricanes or tornados.