Mark the build site by driving a stake at each footer location based on the dimensions of your screen room. The plans of your screen room should show how many footers you will need to dig and where to dig them.
Dig holes that flare at the bottom with a clamshell digger at the location of each stake you drove earlier -- the bottoms of the holes should be about one and a half times the width of the tops. Depth is determined by the frost line for your area -- the hole needs to be 12 inches below the frost line. Check with your local building inspector to see if you need permits and to see how deep you need to dig.You may need to follow up with the building inspector so he can check the depths and give final approval before you pour cement -- discuss this during your initial meeting.
Drive a cement form tube into the hole so that a 2-inch length of the tube is sticking out above the hole and the bottom of the tube is 12 inches above the bottom of the hole.
Work with a team and instruct someone to hold the form tube in place while you build a solid square box around the tube out of 2-by-4 planks. Each plank must come in contact with the tube. Drill screws through the planks into the tube to prevent it from sliding down the hole while you pour the cement.
Mix the dry cement mix with enough water until it reaches a consistency for easy pouring -- about the consistency of a very thick milkshake. Do your mixing in a wheelbarrow for easy transport.
Pour enough concrete into the hole to fill the bottom of the hole and up to the lower 2 feet of the form tube. Agitate the concrete with a long pole to drive out any air bubbles.
Fill in the hole 2 feet at a time with concrete. Agitate the concrete every 2 feet to get rid of air bubbles. Repeat the process until the form tube is completely full.
Use a 2-by-4 plank as a screed and drag it across the top of the form tube to level off the concrete. First drive rebar down into the hole if required by your local building codes. The rebar must extend down to the bottom of the hole.
Install post bases if they are part of the screen room plans. Allow the concrete to cure for 2 days. Spray the concrete with water from a garden hose occasionally to prevent it from drying too quickly -- the concrete should be wet, but not completely saturated with lots of pools of standing water.