Building the garden
The two 2x12 eight feet long will make a four foot by four foot garden. 2x12s ten feet long will make a five by five, and so on. It really just matters that you start with two boards the same length. Treated boards are made with different chemicals lately, but none of them can be very healthy. Regular pine or spruce would do fine for your garden walls. You don't really need a tape measure to find the mid point of the boards. A string the length of the boards folded in half will get you close enough to mark, then check and adjust.
The 2 cuts
The mid point is where you are going to cut the boards so you will wind up with four boards the same length. The handle of nearly all handsaws have a shape that allows you to mark a square line. The frame is easier to put together if all the ends are reasonably square. If you put your palm down on a table top and lift up your thumb, you're doing something a lot of old time carpenters could not do. They had carpenter's thumb. They got it because a handsaw can jump out of the kerf and across the top of the thumb holding the wood, cutting a tendon. The point is, anything designed to cut wood will easily cut flesh. Keep your other hand out of the way as you use your saw.
Once you have the four boards, put on some safety glasses. Make sure you lap the four corners the same way and nail together with a hammer and 16pny nails. Once you've made the box, use the string to make sure the two diagonals of the square are the same. Don't mean to nag, but a career carpenter has a one hundred percent chance of missed days of work due to injury. Safety matters because flying nails will bounce off all of you except your eyes.
Plow the field
Line the bed with the landscaping cloth and fill the space with dirt. If the space is four feet by four feet one foot deep, that's 16 square feet of dirt needed. The seeds are what matter the most, I think. We are being a bit adventurous with our planting, but we're not limited to vegetables. It can be any kind of garden. Maybe we'll build another for an herb garden. Fresh herbs go a long way in a good meal.
two 2x12 eight feet long
string and pencil
hammer and nails
landscape cloth and dirt