Here I show one straightforward way of making small cardboard boxes. There are lots of other ways of making cardboard boxes ~ I will provide notes on some others when I have time. These notes apply to one of the kits we have prepared. It is available on sale in our shop.
Many boxes are made up of two separate pieces ~ a base and a lid. Combining these pieces makes a tray. Making a box is the same as making two trays ~ of slightly different size. Here we concentrate on just one tray ~ with links to other stuff (....sometime...).
Board of some thickness is important for strength, and also for learning some of the techniques required. The thickness of the board allows for butt joints to have sufficient strength to withstand the subsequent covering process - which is always necessary for any type of cardboard box. The cardboard used to package cornflakes is too thin for this technique. It needs to be over 1mm thick ~ over 2mm it becomes very hard to cut. It is made in thicknesses up to 3mm, but that is not easy to obtain.
We suggest ~ and supply in our kit ~ a covered pulpboard of about 1.5mm thickness . Pulpboard is easier to cut than greyboard  or millboard . The method I describe here does involve a lot of accurate cutting. A sharp craft knife ~ a good steel straight edge ~ and mature skills are required. The pasteboard is produced to fine tolerance and is covered during manufacture on both sides with a lining paper. Compressing this sandwich during manufacture creates a board with fine thickness tolerances.
As always I suggest that early projects be treated as exercises with small amounts of material to gain experience. Exercises also need to be useful in some way. All my measurements below relate to a tray-type of box suitable for taking C6 envelopes ~ a common size that takes a sheet of A4 folded twice. Everything here can be scaled up or down ~ I would suggest you save that for later.
The base is the first item to be prepared. A C6 envelope measures 162x144mm. It is necessary to allow about 5 or 10 mm extra to enable a comfortable fit ~ leave tight fitting boxes for later. Possibly one of the dimensions could be increased to 15mm to enable fingers to fit into the box when removing items. I suggest 17cm x 15cm. I have changed units here. The implication is that the size might vary by a few millimetres above or below. Once this size has been determined by you there is no further room for varying dimensions even by fractions of a millimetre. Exact working is important from now on! It is common practice ~ nowadays ~ to stop writing the dimensions when they are understood. So from now on all dimensions are in mm ~ unless otherwise stated.
Cut the base to 170x150. Write these figures down in your notebook. I will explain why ~ later. Check that you have cut the base to be exactly square. If not then you can trim it a bit ~ I have allowed some spare ~ and amend your notebook.
Decide how deep you wish to make your tray. I am working with 2cm ~ 20mm. You now need to prepare two long-edge strips of size 170x20. Then make two short edge strips of 16cm by 20mm. These are slightly over length ~ they will be trimmed, later. (The 16cm indicates that this dimension is not critical.)
There are several ways of assembling the boards you have prepared. Here I am using an approach that covers the tray ~ decoratively ~ at the same time. I think it is unusual ~ but well worth a try. This technique has a number of advantages ~ more on that later.
Prepare a piece of thin ~ strong ~ handmade paper that must be of size at least 12cm larger ~ in each direction ~ than the base. You can be generous about this ~ we will be trimming it ~ later. (That is why the dimensions are stated in cm.) We are using a handy craftperson's technique of avoiding absolute measurements whenever possible ~ we shall ~ later ~ take exact measures from the work being done .
Measure off the position of the base to be central on your piece of paper. Make some light pencil marks. I prefer arrow heads ~ three are sufficient .
Using a suitable glue spread it thinly over one side of the base. Place the base down ~ aligned to your marks. Turn the base to be paper side up ~ gently press down and smooth with a very light finger touch. Do not let the paper slip ~ nor attempt to move it around. If you are off-target remove the whole ~ before the glue sets ~ and try again. If you find that being on-target is difficult then use paste ~ or a diluted glue. After light finger pressure place some chip paper over and press down ~ quite hard ~ finally rub over with a folder ~ which equates to very hard pressure.
Depending on the glue used you might wait a few minutes. For most adhesives ~ glue stick  or normal PVA  ~ no waiting time is needed after rubbing down. For paste  ~ or thinned PVA ~ or EVA  ~ a few minutes ~ waiting for the adhesive to stiffen ~ will suffice. Use the time to do some checking ~ and even more rubbing down will never go to waste. It is always best to rub down  through a protective sheet of clean paper or chip paper .
Lay your base down on a piece of chip paper, with the freshly glued paper side down ~ the inside of the tray is facing up. Take one end piece and check that it fits neatly at the end ~ where it will eventually be. If it was correctly cut all will be well. Paste one side of the side board ~ it will be the outside ~ and run a thin strip of glue along the edge of the base ~ allow a little to drip onto the paper at the edge of the base board. Press the side piece against the base ~ hold it in position for a few seconds ~ pressing down onto the paper and in ~ towards the base. Then take hold of the chip paper and fold it ~ together with the handmade paper ~ up the glued side of the side piece. Keep the base board and the side board pressing down onto the work surface~ the chip paper will want to raise it a little. Check all the time that the side is standing up ~ at a right angle to the base and has not slipped out of position. Use the fingers to gently make sure the paper is stuck all over the side board. Leave it alone long enough for the glue of the butt joint to set.
Repeat for the other side. Wait for a good while ~ overnight if possible. Offer up one of the end boards to its final position and set one end of it exactly to lay at the corner edge of the side board. Make a careful knife mark ~ close to the base of the short edge of the base ~ at the other end. Square off from this mark and trim the end down to the exact length. Check that it is also correct for the other end of the base, and trim the other end board to match. Set them aside for a while and prepare to trim the handmade paper sheet.
You are going to make flaps to wrap around the side joint corners. These will be an extension of the long edges. The flaps will strengthen the end joints, and ~ importantly ~ ensure that there is no chance of the inner boards ever showing through at the corners. Place a straight edge along the long edge of the sides and make a pencil mark ~ along the straight edge ~ at each of the short ends. Repeat at all four corners. You are not going to cut exactly on this line. You will see that the ends are almost ready to fold up and over and into the tray ~ except that if cut on the line they will be too wide to fit inside the tray.
Several reasons. Firstly it is strong ~ even if thin ~ and strong in all directions because ~ secondly ~ handmade paper does not have a grain. Machine-made decorative papers require a tricky technique involving glue or paste onto the paper instead of onto the box.