The footings are the first step. By now you have already designed your deck and have located the footings you require. Spray paint, string, and a measuring tape will help you properly locate the footings. Dig a hole in a bell shape to a depth of 24 inches. In the bottom of the hole lay four pieces of re-bar tied together in a square shape. Shim it off the ground with small stones so it is a couple of inches of the bottom. Now vertically drive two more pieces of re-bar into the center of the footing location. You are ready for concrete.
I like to mix a couple of bags of premix concrete and pour into the footing hole evenly covering the re-bar. This will be the base of the footing. While the cement is wet push in your 8 inch sauno tube into the required location. I always make the tube longer than I need at this stage. Repeat this step until all the footing you have are done and let set for 24 hours. Now you can level the height that your tubes. You need to cut the tubes for your beam to be level. Working on one footing at a time, top up the tube with concrete and set the proper galvanized beam sattle aligned for the beam. For decks above a couple of feet off the ground you will use a 4X4 presser treated post and then top up the concrete. Make sure your tube hieght is above the ground level making sure the post does not touch any ground. This is key to prevent rot. Apply a bead of roof tar around the post and concrete to prevent water penetration.
Now you are ready to install the beams. Check the span codes to be sure your safe. A 3 ply 2X12 is common and you should laminate them with wood glue and 2 1/2 inch deck screws. I install them a little longer than I need so I can cut them to length after the deck joists go in. Attach the beam to the footing or the posts on the footing. Before you lay the joists flash the beams with aluminum or galvanized flashing to prevent water getting in between the laminated beam. Depending on your design you either have 2 beams and you are cantilevering the deck up to the house or you have 1 beam and you are attaching the deck to the house. For the first option, lay and square the 2X10 perimeter joists first and attach them to the 2 beams with 3 inch deck screws. Now fill in the rest of the joists at 16 inch centers. Block framing your joists is always recommended. For the later you will need to attach your perimeter joist directly to the house. This means you have to remove any siding in the way and attach directly to the house frame work. Now you can attach your perimeter joists and fill in the rest of the joists at 16 inch centers. Now you have a solid substructure to finish your deck surface and railings. There are many options to finish the surface and railings. What ever you choose you will have a deck substructure to last a life time.