Step1
Determine the size
You will need 24 inches max per person around the table. In this example, the table is 10 feet long and 4 feet wide. This gives 28 feet around the outside, so it can easily seat 14 to 16.
Since normal sheets of OSB or plywood are 4' x 8', I suggest a 4 feet wide table and then between 10 and 16 feet long.
If you need an 8 foot long table, just use one sheet and two tables.
Step2
Gather the materials
Get the OSB or plywood at a home center. I perfer OSB, since its very cheap. Also buy the firring strips there. You can use just about any material the is one inch thick.
You can use either small platic folding tables or card tables.
Step3
Cutting the wood
Cut the plywood (or have it done) to half the length of the table. If your table is 10 feet long, cut each sheet to 5 feet.
Leave 2 of the firring strips 8 feet long, so they can span both sheets of OSB. Cut the rest to go around the edges of the table.
Step4
Assembling the table - diagram 1
Place the folding tables to one is at each end and one in the middle of where you want the table.
Set the plywood sheets on them as shown. Make sure the center table is supporting both sheets equally.
Step5
Assembling the table - diagram 2
Screw the firring strips around the edge of the table on the under side. These do two things: hold the plywood pieces together, and strengthen the table. Once in place, these will give the table a solid feeling.
A couple of clamps help a lot here, or you can get someone to hold them for you.
Step6
Assembling the table - diagram 3
Here is a sideview of how the table is set up.
Step7
Set the table