ou are usually best to plan ahead when starting a woodworking project. A good set of plans with dimensions, lots of tools in the workshop, pencil and ruler are commonplace. You would never think of taking on a project without those! Well, you can make a medieval box with just a handsaw, hammer, chisel and pencil. If you want to use mechanical fasteners you may need a drill and a screwdriver. For raw materials, you need a wide board (10-inch or more, or have glued smaller ones together) that is around 8-feet long. The edges of your board need to be finished and it needs to be an even width along the entire length.
It doesn’t matter what size board you have, but wider is better. Let’s assume you have a 9-1/4″ (235mm) (W) wide board, that is 3/4″ (19mm) thick (T) and 8 feet (2438mm) Long (L). Boxes are easy to make if they are as wide as they are tall. So the end of the box will roughly as wide as it is tall.
First, we will cut the sides. If you have two boards, overlap them to mark the width. Mark and cut on the outside of the line, and then mark and cut again so you have two square boards. If you only have one board you can use a stick or piece of paper to mark the width. Be sure to cut on the outside so the line just barely remains. 
We use some ‘fancy’ and strong joinery (simple finger joint) to make the box. Make sure the grain is running top to bottom on the side and mark the sections to cut out. They will be equal to the width of the board. You can mark the width to cut with the remaining board length. The pieces to cut out should be identical. The middle section remains and does not have to be the same size. Usually, a rule of 1/3 works out well. You should be cutting on the outer edge of the line, so it just barely remains after sawing. 
Use the height of the board to mark the depth of slot cuts. 
Now we can cut the top, bottom and sides. Divide the remaining board into four equal lengths. You can do this with a stiff piece of string and fold it in half two times to get 1/4 lengths. Mark one side with a B and a T for bottom and top. Make the bottom one board thickness shorter than the top. Cut all four boards along the lines. 
Take the end boards and lightly mark the side cuts you made. Do this on both sides of the board. Note that those two pieces actually remain and it is the middle section that gets cut out. Put an X on the centre section so you don’t make a mistake when cutting. 
Cut out the inner section staying on the inside of the line. Do the same at the other end of the side and the other sideboard. 
Use a saw to cut the slots and a chisel and hammer to remove the middle piece. (You could also use a coping or bandsaw) 
The two sides should look like this. 
You should be able to put it together at this point. Make sure the best surface of each piece is facing out. You will likely need a chisel to make the joinery fit snug. As you are fitting the pieces, mark the corners so you know which end belongs to which corner. It is unlikely all your joints will be perfect, so each one is unique at this point. 
Find a surface with a 90-degree corner and glue and clamp the box. Make sure it is 90 degrees. I will often cheat and use 2″ wood screws with 1/4″ countersunk holes, then use 1/4″ plugs to make it look like dowels were used. If you like you can make medieval nails from common nails by hammers the heads square, drill pilot holes and do it that way. You can also just let the glue do its job and don’t both with dowels. 
You only need a few nails/screws/dowels. 14 should do the whole box. Make sure the box stays flat on your tabletop. Use weights or clamps to make sure it stays flat while the glue dries. 
Now you can cut the bottom of the box from the shorter piece you marked with a B. Your box, despite your best efforts, probably isn’t perfect and the corner angles aren’t 90 degrees. You may have miss-cut one side. It’s OK if it isn’t perfect. If you trace the inside of the box onto the bottom board, you can then cut that out on the outside of the line and you will have a perfect fit. 
I usually do the nail/screw/plug technique for the bottom, or you can just glue and clamp. 
The lid for the box is made the same way as the bottom, but you trace the outside of the box rather than the inside. 
You will end up with a box similar to the one below. It wasn’t made from a single plank and the joinery is backwards from the plan, but it gives you the general idea.