I have only read the All New Square Foot Gardening Book and I am pretty sure it goes into detail on how to construct the boxes. I'll give you a quick run down though.

No the boxes do not need a bottom. Some people do and some people do not. If you are going to go the 6" deep route I would definately go without a bottom because the roots will most likely go down a little more. Go to one of the box stores and buy two 8' pieces of 1x6 pine. Cut the pieces in half. Now you have 4 4' pieces of 1x6. Screw those together into a box and fill with Mel's mix or other soil. The grid doesn't have to be anything special. You can just eyeball it or actually mark it with just about anything. There is a thread on this board that goes into detail about what others use to mark them. Everything from old blinds to bamboo to string to lattice work. Some people fix the markers to the box and some just lay them in the dirt. Do you have the book?

I also have Mel Bartholomew's book, All New Square Foot Gardening. It is a great resource and explains in detail how to build the boxes, fill them, add the grid, and how to plant.

However, I learned the most from reading the posts on this forum. There are also a plethora of instructions on the web. Try googling "SFG" or "Square Food Gardening."

You can also learn a lot here Official Site of Square Foot Gardening and Mel Bartholomew

Rachel
Graphic Muse's Garden Spot

if ya don't want weeds coming up ya can use on the bottom, landscaping fabric (found at lowes or home depot etc.)
or card board, make sure they are overlapping, or you could use several layers of newspapers. one lady told me to use 6-9 layers. worked great!. the last two will decompose. :') ~Medo

and if you use cardboard or newspaper instead of landscape fabric, the earthworms can get in.

Did you also see the thread posted by jj_upstateny with building instructions?

Here is a link that might be useful: Instructions thread